ULTRAVIOLET STERILIZATION (How UV sterilization works). This article explains the benefits and myths about aquarium and pond uv sterilization

	This is easily the most important simple aspect of proper UV Sterilizer set up and all the gimmicks (like “twists” and “wipers”) will not help a poorly installed UV with high flow rates of water passing through. In fact this is where most UV Sterilizers installed in ponds fall well short as pond keepers will place for instance a 9 watt UV with a 1000 gph pump which is 2-4 times the effective flow rate for a pond installation (often as per very poor directions by the manufacturer). This problem is also common among reef aquarists with high flow rate sump pumps attached to UV Sterilizers incompatible with the flow rate, resulting in the common and incorrect statement that “the UV Sterilizer was useless”. Admittedly flow rate is a generalization expressed to simply mW/cm2 that is absorbed/transmitted to pathogens/algae (please read further in this article for more in depth explanation of UVC Penetration/mW/cm2). I use this method as simplified method that has tested well over the years in my applications of UV Sterilizers. Obviously there are other factors such as distance from the UV wall, water turbidity, quartz sleeve cleanliness, UV bulb condition/age, and more (again that are explained in this article). As well many UV manufacturers will over state their UV Sterilizer abilities as per maximum flow rates (although this does not necessarily make such a UV Sterilizer a poor/inferior product as some other web sites claim, I would only use the simple calculations provided here to make your choice, not exaggerated claims by many UV manufacturers). • The key to success is CORRECT water flow, cleanliness of the bulb and/or quartz sleeve, water particulates and water temperature. Please also see the table at the end of this section that further explains flow for Sterilization or clarification for aquariums, ponds or even pools Also please note that the given flow rates are generalizations, with many Compact UV Sterilizers requiring the slower flow rate given over a Standard length UV (such as the TMC Pond Advantage/Vecton) *As a Sterilizer; Generally for bacterial control (& many virus) a flow rate of 20-25 gph per watt (75-95 liters per hour, per watt). *As a Clarifier; For algae control, 40 gph per watt (sometimes as high as 50 gph) is effective to maintain effective exposure for effective UVC sterilization/radiation (depending on model UV’s design). *As a Sterilizer; For single cell parasite control (such as Cryptocaryon) as well as a few “stubborn” viruses, a flow under 10 gph (or even less) is necessary. This is often not 100% for all parasites of this type, so a UV Sterilizer should not be relied on as the sole preventative for these parasites! Please see the section (“What Size UV Is Best”) further down in the article for more about flow rate. Admittedly this is a VERY basic and simplistic rule, with other factors such flow design, interior wall gap from UV bulb, & even actually output of UVC per watt (Versus other wave lengths). *The design of the unit should only allow a gap of .3 cm or less between the bulb or quartz sleeve and the wall of the unit for effective contact and temperature. This is where canister aquarium and pressurized pond filters (such as the Aquael, Nursery Pro, even the newer Via Aqua) fail miserably as there is too large a gap and in general a poor flow design around the bulb/quartz sleeve (as well as flow rates far exceeding UVC bulb wattage capabilities) • When figuring the flow (gph) of a water pump, consider the flow of water AFTER it passes through the UV clarifier and reaches the aquarium or top of water feature in a pond. For instance water pump or filter rated at 400 gph at 0 head pressure & 6 feet maximum head pressure (which is how all pumps are rated), will likely only push about 200 gph after being lifted 3 feet from a sump or up a water feature. For a positive check of flow rate, simply use a stop watch (many cell phones have this feature), and place a container under the outlet. For example if one gallon is filled within 10 seconds, this is 6 gallons per minute or 360 gph. Many persons are surprised how slow their pump actually is after applying head pressure. Some pumps or filters lose head pressure more quickly than others (such as the Hagen Fluval), while other pumps are specifically designed to maintain higher head pressure over greater vertical distances (such as the Rio HF Series pumps). Size of pipe/tubing on water flow: Another consideration is the size of the PVC pipe or tubing coming from the pump. For instance a pump with a 1 inch outlet port that is rated at 2000 gph (for 0 head pressure) will likely not have an output of 2000 gph if the pipe/tubing used is ¾ inch or less. This same thought goes for the UV Sterilizer itself, in fact the Custom UV I have built only uses ½ inch hose barbs so as to restrict the flow to under 350 gph which is the maximum effective rate. It is also noteworthy that the flow rates I have already noted (20-25 gph per watt for sterilization or 40-50 gph per watt for green water control) is not an exact rule, as reflection within the UV Sterilization chamber, the distance between bulb (or sleeve) and the walls of the unit, and the length of the bulb as per wattage of UVC output all affect this general guide (this is explained later in the article in more depth). The point as to bulb length is noteworthy, as I have found that increasing wattage with the same length bulb does NOT increase effectiveness proportionally to a given wattage. In fact a popular Pond UV that uses HO (High Output) UVC Bulbs (50 watt and 100 watt models) does not double the amount of water flow you can run through the unit even though it is double the wattage of comparable length UVC bulbs of half the wattage. The TMC 110 Watt Pro Pond UV is much more effective than the popular HO UVs of 100 watts (each use two bulbs, the TMC uses two 55 watt of 36 inches while the “smart” HO UV uses two 50 watt bulbs of 18 inches), yet these HO UV Clarifiers cost much more which often leaves me scratching my head as to “why?”. Diverters/ By-pass Valves; For pond UV applications (or high flow rate aquariums such as sump systems) it is usually best to have a ‘T’ with a ball valve to split the return line to the pond from the filter, this will allow the pond keeper to slow down the flow to the UV to the correct rate as per gph and wattage. (Please click on the picture to the left for an example of a UV diverter with a control valve and also to purchase them). Aquarium or Pond Water Turnover Rate: It is important to have a flow rate that will turn over the pond or aquarium at least once or 1 ½ times per hour for disease prevention (regular Sterilization or up to 4 times for stage 2 sterilization) or every 2 to 3 hours for green algae control (I have achieved effective disease sterilization as tested with a bacterial cloud with as little as once per two hours with a good installation in an otherwise well maintained aquarium). Example: 100 gph will service a 100 gallon pond well. The reason I mentioned such seemingly slow rates is too high a rate will not usually allow for adequate contact time with UVC radiation. In ponds, again I want to emphasize that many manufacturers will make claims of certain UV Sterilizers/Clarifier per pond size; however the flow rate through the UV and turnover rate is more important. As well many will correctly install a UV Sterilizer for their pond, ignoring the importance of good filtration combined with UV Sterilization. A correctly installed UV Clarifier often will not over come poor filtration and circulation. Please read the article below for much more abo

Flow Rate & Turnover Rate Table:

UV-C Use Flow Rate (generalized) Turnover Rate
Clarification
(Green water control; Aquarium/Pond) 45-50 gph per watt Once per 2-3 hours
Level One Sterilization
(Bacteria, some Virus) 20-25 gph per watt 1.5 times per hour
Level Two Sterilization
(Parasites, “Stubborn” Viruses; Recommended for Swimming Pools) 8-10 gph per watt Up to 4 times per hour
TMC 440 Watt Industrial UV Sterilizer
As you can see, if level two sterilization is required (as is generally necessary for swimming pool use as well as most “high end” marine fish importers such as Quality Marine USA), the need for large UV-C Sterilizer may be necessary for these large volumes of water such as the TMC 440 Watt Industrial UV Sterilizer pictured to the left.

For a further explanation of the subject of gph & turnover rates as it pertains to the correct UV for your application, see the section “What Size UV Sterilizer is Best”

Temperature/Quartz Sleeves
Generally UV Sterilizer Bulbs used in aquatics employ low pressure mercury lamps (39% efficiency) vs. the medium/ pressure mercury lamps that have a much lower effectiveness (10%).
This is important to note as low pressure lamps are sensitive to water temperature while medium pressure are not (the poor efficiency of medium pressure lamps which produce much of their light in spectrums useless for sterilization makes this only quality negligible). The operating temperature spectrum at which low pressure UVC is effective is between 20 C (68 F) and 40 C (104 F). The keyword is OPERATING temperature as in a reasonably well designed unit the water is usually in the unit long enough to be warmed several degrees which generally means that a water temperature over 20 C (68 F) will work fine. So if your flow or unit design (or both) is not allowing this temperature, your effectiveness will be minimal.

Quartz sleeves help keep up a higher temperature, however in my tests with the same wattage units, (with and without Quartz sleeves) I only found a 5 degree F difference inside the UV Sterilizer when I turned off the units and then checked temperature differences, so this is another one of those statements about UV Sterilizers that I will challenge.

Quartz glass is also used in some UV Sterilization applications to protect a UVC light that is suspended above the water from spray (assuming the quartz glass is kept clean of mineral buildup). As per the latest data I have; quartz is about 99% efficient as per UVC penetration, so it is the best product for this use, however polycarbonate plastic is about 90% efficient, so this could also be used. I do not recommend acrylic (about 60% efficient) or standard float glass (about 40% efficient).

Although useful in some applications, quartz sleeves don’t always allow as good of contact with the water and are difficult to keep clean, so in my opinion the benefits are a wash. Which brings me to key # 3, keeping the bulb or quartz sleeve clean?
First, you want to change the UV bulb every 6 months for disease prevention or once every year at the beginning of the growing season for green algae prevention. You want to clean the quartz sleeve at this time too (using acetone), which is one reason for a simple model without a quartz sleeve, use just throw away the dirty bulb, although the temperature aspects of a quartz sleeve are worthy of note.

Be careful of the latest gimmick in UV Sterilizers and that is wipers (great in theory though), for more about these, please read this blog post: "UV Sterilizer Truths".
I would also be aware of many other sources of inaccuracies in UV Sterilization, please read this blog which has regular posts dealing with these subjects: UV Sterilizer Blog

Back to temperature; if your water is very cold (such as a pond or even well water treatment system), I would suggest pre-heating your water before it passes through your UV Sterilizer unit, this is more effective than a quartz sleeve.

For further information about UV bulbs (lamps) and how they work, please see this article:
UV (UVC) Lamps (Bulbs) used in Aquariums and Ponds and how they work.

Aquarium Temperature with UV Sterilizer Use:

The section above deals temperatures inside the UV Sterilizer itself, not an aquarium, Please do not confuse the two as although the temperature inside the aquarium (& more likely a pond during cooler months) affects the effectiveness of the UV Sterilizer, the UV Sterilizer has little effect on the aquarium temperature.

For example, a 9 watt UV Sterilizer connected to a 100 gallon tank can at best heat your tank the same as a 9 watt heater.
With a heater 25 watts is required for every 10 degrees of ambient temperature per 10 gallons you need to raise your aquarium temperature (from Heaters; Aquarium Answers); so using this equation a 9 watt UV-C lamp (again at best) could only raise 10 gallons of water 3.6 degrees F; now divide this by 10 (since a 100 gallon tank is tens times then gallons) and you have a temperature increase of .36 degrees Fahrenheit, in other words insignificant!

UV Transmittance; Other Factors Affecting UV Sterilization

Another important factor that is often overlooked and sometimes controversial (as per my reading of some forums, especially reef forums) is the water’s UV Transmittance. UV Transmittance which is expressed in percent (0-100%) of UVC Transmittance.
This measured value is the difference between a known UVC light source and what is measured by a calibrated detector through a 1 cm thick sample of the water to be irradiated/treated.

Water turbidity (dissolved waste particulates, DOC, etc.) in the water column, poor circulation (a poor flow pattern that that has the water pickup and return too close where too much water goes unfiltered). If these factors exist and more UVC light is absorbed/blocked you will need a larger UV Sterilizer for an effective pathogen kill or green water control

Particulates in the water column interfere with the effectiveness of a UV Sterilizer by blocking UVC light and absorbing some of this energy.
For this reason, the UV Sterilizer is best placed AFTER a filter so as to remove as much particulates from the water column as possible (the UV Unit is also best placed on the exhaust/return form a filter so as to allow maximum nitrifying bacteria accumulation in a bio filter).
Sometimes this is not always possible or desirable (as per some reef keepers that that feel that pre-filtration increases nitrates), in this case you MUST increase the size of your UV to compensate.

Time; Generally a UV Sterilizer is best run 24/7, as this will provide the best Sterilization, Algae Control and Redox improvement. In Ponds this is particularly important as it can be difficult for a UV Sterilizer to keep up with algae growth during peak sunlight hours, while nighttime allows for a UV Sterilizer to “catch up” with algae spores. In Redox balance, a UV run constantly will provide a more stable balance.

There are exceptions though; many in Reef aquariums do not run a UV during certain hours (often using a timer) such as feeding micro planktons.
A UV Sterilizer should be turned off when certain antibiotics are added (see the next section), when seeding tanks with established filter media, or Autotrophic or Heterotrophic Bacteria are added for bio waste composting or cycling (such as cycling products like SeaChem Stability). Generally 24 hours off is long enough for these products.

UVC Penetration (Microwatts second/cm2):

The emission or light intensity of a UVC germicidal light bulb is usually expressed in a term called "microwatts per square centimeter" (Mw/cm2). The maximum intensity provided by a single UVC Bulb is at its surface. So, if we calculate the surface area of the UVC lamp and only use that area which effectively emits UVC light rays, the effective area of UVC transmission will be established. Basic mathematics will show that the surface area of a cylindrical tube is ‘pie’ D L.

Next extrapolate this effective area of UVC transmission as having a screen with squares 1 centimeter in size. Each of these cm2 areas now, for measurement purposes, emits a UVC lamp intensity measured in microwatts, in other words; the term microwatts/cm2. UVC light intensity decreasingly varies as the distance from the UVC light increases.

Put more simply (a non scientific analogy); The amount of wattage will also increase penetration, as a higher watt UVC bulb will generally have more Mw/cm2. In my own experiments I have used 15 watt and 25 watt UVC bulbs in exactly the same unit (both were 18”), if wattage were only considered there would be a 60% increase in effectiveness, however I only observed a about a 25% increase. When I used a 30 Watt UVC bulb in a unit with over twice the exposure as the 15 Watt, the kill rate more than doubled. From my experience, if you increase wattage (and Mw/cm2) you need to also increase the volume of water to maximize the higher watt bulb.
Experiments can also be safely conducted with standard household light bulbs to correlate penetration. For this start with a 7 watt clear bulb (such as a Christmas bulb) and place varying thicknesses of paper/ cardboard in front of the bulb and measure when penetration stops. Continue this with higher and higher wattage bulbs.

UVC Intensity The Diagram to the left can give a rough comparison of distance as per UVC energy as expressed by MW/cm2 in Air transmission.
The dose applied by an UV-C lamp installation is a function of the lamp output, the intensity factor, and time. As an equation; Intensity x Exposure time= microwatt seconds/cm2.
As an example, a 9 watt UVC lamp at one inch from the lamp is found by this formula:
9 x 127 = 1143 mW/cm2.
Since many bacteria such as Vibrio require a UVC exposure of 6500 mW/cm2 or more, this means an exposure time of 5.68 seconds is required to kill this pathogen


Now let me point out that even though I have published this diagram, please use this as a rough guide only, as I have found inaccuracies in it. To be more blunt, I have found the distance, wattage, and flow rate to be the MOST IMPORTANT factors in determining exposure/effectiveness. This diagram is STATIC and does NOT take into consideration the dynamics of UVC radiation penetration for which I have yet to find a good formula to demonstrate this (even in University studies). Most standard low pressure mercury lamps used in UV Sterilizers have a surface mW/cm2 of 36,000 to 42,000 and the VHO (high output) between 50,000 – 72,000 mW/cm2 depending upon flow rate.
The generally accepted mW/cm2 is 30,000 mW/cm2 for Clarification and up to 90,000 mW/cm2 for Sterilization (depending upon the pathogen/parasite in question)

What is often missing in any equations I have seen is the dynamics of water flow geometry, actual water flow, and wattage. The bottom line is to use this table and others you might find elsewhere with “a grain of salt” noting that these are static and even then are flawed when true output via wattage is taken into consideration.

For more on this subject, please see this article:
UVC, Watts, Microwatts, Joules, & light penetration

Penetration Capabilities:

UVC rays do not have great penetration ability on most substances. In the case of air, UVC has a very effective “killing” range. In the case of water (which is what we are dealing with), this can vary considerably with dissolved solids, mineral content, organic content and more. This is why pre-filtering water is important for proper UV Sterilization. Your UV Sterilizer should ALWAYS be connected after the filter, not before. Water that is still laden with minerals, organics or more will need more UVC radiation in terms of Watts and time exposure. Agitation of water being sterilized increases sterilization effectiveness.

Here is an interesting article (and experiment) demonstrating UV light presence:
Demonstrating UV light presence, UV sterilizationDetecting Ultraviolet Light Using Tonic Water


Here is a blog about UV Sterilization (much of the same information, however I have regular posts with new information especially answering to much of the misinformation out there): Aquarium and Pond UV Sterilizer
This is good blog to bookmark as I will often add information here not found in this article so as to keep this article reasonably easy to understand.

Effect of UV Sterilization on minerals, chemicals, etc.

This is an area where a lot of information is posted in aquatic forums and similar that has little scientific proof to back up statements.
I myself have noted the affect of UVC radiation on a variety of chemicals added to treat infections and I have noted a variety of end results, HOWEVER I also have not conducted hard scientific studies nor have I found any reputable studies to make conclusive statements (although many persons do without much to back themselves up).

What I have observed is that there is no affect of UVC radiation on minerals, and many chemicals such as Malachite Green (despite comments that UVC breaks down Malachite Green I see no evidence as of writing this).
I however have seen UVC radiation breakdown some light sensitive antibiotics such as Quinine Hydrochloride.

Here is a list of PROVEN light sensitive chemicals: Bromine, Oleic Acid, Ethyl Ether, Potassium Ferricyanide, Ferric Ammonium Citrate, Silver salts, Hydrobromic Acid, Sodium Iodide, Mercuric Salts, Mercurous Nitrate.

Reference: Light-Sensitive Chemicals


WHAT SIZE UV IS BEST?

For sterilization (level 1) you want 20-45 gph per watt and an aquarium turnover rate of 1-1/2 times per hour (recommended minimum).

Example: In this example I will use a 100 gallon aquarium;
*For Clarification (Green Water Control; generally the most common application in ponds) you would need a flow rate of 45-50 gph and a turnover of aquarium/pond water through the UV of once per 2-3 hours. So as a minimum you would need a flow rate of 50 gph (assuming once per 2 hours in water turnover). At 50 gph the smallest UV available is a 5 Watt and this would more than provide the 45-50 gph per watt (actually this would be 10 gph per watt).
*For Level One Sterilization (generally the most common application in aquariums), you would need a turnover rate of 1.5 times per hour which would be 150 gph for this example. The flow rate through the UV should be 20-25 gph per watt (I will use 20) which would mean 7.5 watts would be the minimum UV Sterilizer.
*For Level Two Sterilization, you would need an aquarium/pond turnover rate through the UV Sterilizer of up to 4 times per hour, which would mean 400 gph for this example. The flow rate in gph per watt is best under 10 (generally even lower, we will use 8 gph per watt for this example), so at 400 gph this would require a 50 Watt UV Sterilizer.

These examples also due not factor in the design of the unit as per gap between the bulb/quartz tube and the wall of the Sterilizer, as this is can require an even larger UV Sterilizer if the gap is more than .3 cm which is most often the case in many, especially pressurized pond filters and even (albeit to a lesser extent) to popular UV Sterilizers such as the Turbo Twist

Again, please note that these examples are minimums

Further Examples/Suggestions
Multiple units in parallel work well for larger ponds (Ex.: two Terminator 18 Watt UVs will easily handle a 3000 gallon pond if pair with either two pumps with the correct flow rate or one pump of for example 1500 gph that is split two ways (after head pressure, which may mean a pump of 2200 before head pressure, depending on lift and other head pressure factors).

Sometimes multiples of even three which can often provide better circulation for ponds or for simplicity on very large ponds, UV Clarifiers such as the TMC 110 watt Professional Pond UV Sterilizer which can handle ponds up to 12,000 gallons by itself.




Here are basic flow rates for UV Sterilization applications; this of coarse will vary due to UV Sterilization design (not all Sterilizers are equal) and many other factors:

*For algae control in ponds (this basically clumps the algae in the water column for filter removal, it does not outright kill the algae at this flow rate as a lower flow rate is required to outright destroy the algae); 40- 50 gph per Watt

*For bacterial and fungal control as well as aid in Redox Potential; 20- 30 gph per Watt

*For parasite control; 5-10 gph per Watt and sometimes even lower.
(This is nearly impossible at the flow rates most UV Sterilizers are set at so this should not be your primary goal for the purchase of a UV Sterilizer, although the UV will affect other parameters that will aid the fish in resisting parasite infestations).

*As noted earlier the previously given flow rates are generic generalizations that do not take into consideration UV design, water temperature, water turbidity and more. Water that is not pre-filtered is going to be more turbid (higher TDS) and require a lower flow rate, as well the design of some UV Sterilizers often necessitates a lower flow rate as well This is especially true in units with larger water volumes around the UV bulb/Quartz Sleeve as often found in Pond filter with built in UVs such as the “Clear Stream with UV”, “Fish Mate 2000, 3000, 6000” and “Savio Skimmer with UV” and many others (this is not to say ANY of these are bad filters as they are all excellent filters, they are simply not the best way to use a UV Sterilizer in a pond!)

As noted earlier there are many excellent UV Sterilizers/Clarifiers with good basic and reliable designs, however there are many excellent high end UV Sterilizers such as the Matala stainless steel UV Sterilizer that may appeal to those with large ponds with high flows. These may be a worthwhile purchase for many, however in my experience, I have achieved equal results with lower cost UV Clarifiers such as a pair of Terminator 36 watt UV Sterilizers @ $140 each ($280) connected at the proper flow and pond turnover rate as one high end UV Clarifier such as a 75 watt Matala that costs over $800; so be careful on being over sold for your pond clarification needs (as well look into improved filtration including a DIY Veggie Filter)
For more information about pond applications (including a diagram for reducing flow), please see the UV Sterilizer section in this article: A Clear Pond; Pond Information

Aquarium UV installation diagram with Internal Filter or Power head
Here is a basic picture for plumbing a compact UV Sterilizer using an Internal Filter or Power Head. The lines and can be much more neatly placed than the diagram (this is just for emphasis). This demonstration is shown on the front of the tank for easier viewing, obviously this would be placed on the back of an aquarium.
You may also place your power head/ internal filter on its side for an easier tubing application.
The picture can be enlarged by clicking on it too!
This kit can be found here: Terminator Compact UV Sterilizers

Internal Wet-Dry Bio Filter with UV Sterilizer This picture shows an Internal Wet Dry connected to a UV Sterilizer, this application also applies to the similar Bio Cube Internal Wet Dry filters (please click to enlarge picture)

Fluval Filter with UV Sterilizer A common UV Sterilizer Application is to a Canister Filter or Pressurized Pond Filter (these pressurized pond filters make an excellent alternative aquarium canister filter as well).
Fluval Canister Filters have “ribbed” tubing, which can still be used to attach to a UV Sterilizer (using Teflon tape around the UV male hose barbs and hose clamps), HOWEVER I recommend replacing the Fluval tubing with standard vinyl tubing as in the picture to the left.


For a couple of pictures/diagrams as to UV Sterilizer/Clairifier connection to a Sponge Filter, please see this article: “Sponge Filtrations; UV Sterilizer Sponge Filter Applications”

Aquarium and pond tubing installation For help in connecting aquarium tubing to hose barbs or similar connection, please click this picture.










Is Too Large a UV Sterilizer a Problem?

This is an occasionally asked question to which the answer is generally no with some exceptions.
In my view spending more for say a 36 Watt UV connect with a 300 gph water pump flow will produce 8.33 gph per watt, however this does not generally produce much better results than 20 gph per watt. While it may be true that parasites such as Cryptocaryon (marine Ich), are more readily killed at this rate, this is not an established fact either (as of writing this). I find that UV Sterilizers help control Ich or similar parasites not by killing them, but by improving water quality such as Redox which in turns improves the fish’ natural resistance allowing treatments or other measure to eradicate a given parasite infestation.
As well in a marine reef tank in particular, this high wattage to flow ratio may kill copepods or other propagated organisms (assuming they get into the water column, which is rare). While the facts that many of the organisms propagated in marine reef are not killed by typical UV flow rates (20 gph per watt), making it a myth that UV Sterilization should not be used in reef aquariums, I would also not push the envelope with flow rates under 10 gph per watt.

Another aspect to consider with low flow rates per watt (generally under 10 gph per watt), is that I have often found that with commonly used UV Sterilizers that the aquarium turnover rate is affected which is another important aspect of UV Sterilization. With the above example of the 36 Watt UV and a flow rate of 300 gph on a 100 gallon aquarium would not be a problem. However lowering flow rate to 50 gph (so as to maintain 10 gph per watt) with a 5 watt UV Sterilizer would considerably lower UVC effectiveness in this same 100 gallon aquarium

This all said, if you do not have a marine reef aquarium, just a fish or plant only aquarium and feel more secure with a flow rate of 10 gph per watt, by all means go for it, but generally this is just money wasted in my experience.

WHAT IS A QUALITY UV?

Aquarium and Pond UV Sterilizer, Custom UV, Compact Ultraviolet Unit
Proper UV sterilization starts with contact time, water turnover, water turbidity/filtration, water temperature, bio load, and more.
Proper filtration of the water before entering the UV sterilizer helps lower turbidity; many UV’s now available do not have proper contact time or advertise too high a flow rate. I would recommend 20-45 gph per watt of UVC. I have produced a 15 watt UV that is extremely effective when installed properly (in part because it is flow restricted and will not allow more than 350-400 gph).
UV sterilizers that place the UVC lamp above the water (there are several on the market, and I have used them), are generally less effective, again due to poor UVC exposure.


High Performance and value Terminator UV Sterilizers by Via Aqua Be careful of some of the cheap UV sterilizers currently flooding the market such as the Aqua Medic, our service experience is poor with these units (leakage, poor sterilization patterns). If you obtain one of these units, you cannot make a fair assessment of what UV Sterilization can really do.
Also beware of the Internal UV Sterilizer out of China, first by Jebo now called the Killing Machine. We thought this to be an awesome idea ourselves, however after testing they failed miserably with not one of these Internal UV Sterilizer passing durability tests. What is sad is that they are now being marketed as the Killing Machine after most reputable retailers have since rejected the originals.

Effective ones (besides my custom unit) include the Coralife Twist (albeit over priced), the Cyprio, the ReSun, Tetra, the Via Aqua Terminator UV Sterilizers and many others.
Even with many of the before mentioned UV Sterilizers/Clarifiers, many although of good design and reliability, are often over priced or imply features that add no effectiveness to actual sterilization effectiveness. The Tetra Pond UV Sterilizers (such as the GreenFree™ UV Mini Pond Clarifier) although well made adds no more effectiveness than a Terminator 5 watt UV Sterilizer. In fact you will likely pay twice as much for the Tetra over the Terminator only for a slightly thicker plastic housing, and despite some common perceptions, neither should be kept in direct contact with weather and are only water resistant, not water proof. I will also add for pond applications, generally a 5 watt of any brand is too small but for the smallest of water features (I generally recommend 9 watt or larger).

If heavy duty UV Sterilizer construction appeals to you, rather than the Tetra Pond Green Free UVs, I would suggest the TMC Pond Advantage such as the TMC UV-25 Pond Advantage. TMC (Tropic Marine Centre of UK and Germany) is the European leader in aquarium technology, including UV Sterilizers and unlike the Tetra Model you will also be purchasing a HO straight UVC bulb with much better UVC exposure time for a better price the Tetra Pond UV 18.
The TMC Pond Advantage UV25 is the heavy duty version of the TMC Vecton UV25 and is excellent for Aquarium use as well as pond use.

The bottom line is if you are looking for a “High End” UV Sterilizer/Clarifier, the TMC line of UVs is without equal when performance/effectiveness, durability, and price are all considered.

I would be aware of otherwise good Pond Filters such as the Clear Steam Pressurized Pond Filter and many others such as Cyprios, Tetras, as well as some new Aquarium Canister filters that come with built in UV Sterilizers in the top. These are rarely as effective as a separate unit for two major reasons;
*Flow rate is often too fast for proper contact/exposure and rated wattage to generate the correct temperature around the bulb.
*The water in these style units is not contained in a small space around the UVC bulb (less than 3 cm) rather the flow is in a large area around the bulb which is generally not adequate for good UVC exposure.
It is always best to purchase a UV sterilizer that can handle the flow rate as per its wattage or divert the water through a ‘T’ Valve to the UV at a slower flow rate than the Filter or main pump is running at.

Finally be aware that although HO (high output) UVC lights are certainly an improvement over a similar length UVC bulb of similar length, they do not increase effectiveness proportionally as to wattage per length. Often HO bulbs are very short and even with this higher output, these are often still not enough for the water flow that is usually applied to these UVs. For example, although a 25 watt UVC bulb of 18 inches will handle a higher water flow of a 50 Watt HO bulb of 18 inches, the flow rate is not doubled.
Better is a unit with long exposure such as the TMC Professional 110 Watt UV Clarifier as compared a Smart HO Two-Lamp 100 Watt UV Sterilizer. The TMC Pro 110 Watt is not only vastly less money, it is a superior UV Sterilizer in terms of UVC exposure and performance!.

As a side note, I as the author of this article (which not only represents 1000s of hours of research, but many years of practical use and controlled tests) obviously hope to sell a UV Sterilizer that I choose to make available on my web site (of which the mark up is extremely low so as to make them more readily available). This does not discredit the research and experience that has gone into this article, nor does this mean that there are not many good UV Sterilizers available elsewhere (such as Aqua, Emperor, Coralife, etc.). However I choose to recommend UV Sterilizers I would use myself for my clients based on quality AND value (many excellent UVs are over priced for what they provide in capabilities IMO). It is also noteworthy that there are also several UV Sterilizers that are of poor quality that are not worth purchasing at ANY price (I will leave mentioning these to my blogs)

UV STERILIZER MAINTENANCE:

UV Sterilizer maintenance is quite straight forward; make sure you keep your unit dry on the outside, if used for a pond try and protect your unit from harsh weather (most sterilizer can withstand the outdoor environment, they just last long if they can at least be partially sheltered).
Bulb: Change your bulb every 6 months for aquariums, and also every 6 months for ponds in warm climates where there is no winter freeze. In cool climates a pond UV bulb can be changed every season (usually late spring/ early summer)

Quartz Sleeve; If your UV has a quartz sleeve it is important to clean the quartz sleeve when changing your bulb, otherwise your bulb change will not be very useful. Be very careful when cleaning a quartz sleeve as they are very fragile and expensive (and usually hard to find replacements). I recommend cradling the quartz sleeve in a pillow while cleaning and using acetone to remove scum and other deposits (not glass cleaner).
Some UV sterilizers come with wipers which are used keeping your quartz sleeve somewhat clean between bulb changes, however they are more of an expensive gimmick (my experience with them is they do little to remove build ups that block UVC light) and still do not take the place of cleaning your quartz sleeve at bulb change time.

Ballast/Transformer; This part of a UV Sterilizer, Purifier, etc. often does not last as long as the main body/unit itself. This is more true of the electronic ballasts than magnetic (although the starters used with magnetic ballasts often need to be changed every year), as well many of the low quality units that are now flooding the market often have cheap electronic ballasts that last 6 months to a year. Poor design, care, or placement of a UV Sterilizer/Purifier can also prematurely destroy an otherwise good ballast.
The use of a multi-meter is the best way to check a ballast. Assuming a 120 V connection (North America), the output from the ballast should not be 120 Volts A/C, rather is should be reduced. For example (as per my own tests with a multi-meter), a 5 watt UV Sterilizer has an output after the ballast of 16 Volts while a 36 Watt UV had an after ballast voltage of 27-29 volts. This reduction should be the same with a 240 Volt UV designed for markets other than North America.


FACTS (& MYTHS) ABOUT UV STERILIZATION;

Here are few things UV Sterilization will NOT do:

[1] UV sterilization will not cure infected fish of bacterial or fungal diseases. A UV can aid in cure by killing bacterial pathogens in the water column and fungal spores, also by improvement of the Redox potential (which is much more important then many realize based on scientific research) and general water quality.

[2] A UV sterilizer will not kill ich trophozoites already on the fish (but then medications don’t either), but UVC can again slow the spread of ich tomites in the water column (but usually not out right kill ich tomites). However by water quality improvement (such as Redox Potential) and lowering of pathogenic bacteria, the fish has more natural resistance to fight Marine Cryptocaryon or FW Ich.

[3] A UV sterilizer will not kill beneficial bacteria such aerobic bacteria, as this bacterium is effective when attached to a surface of high water flow such as the sponge of a sponge filter, not when in the water column. In fact relatively new scientific evidence shows nitrifying bacteria to be sticky and adheres to the surfaces like glue this is why the myth of UV Sterilizers killing beneficial bacteria is just that, a myth. It still may be best to turn off a Sterilizer unit when introducing bacteria in liquid form to seed a new aquarium.

[4] UV Sterilization will not remove or destroy algae growing on tank or pond sides, rocks, decorations, etc.

[5] UV Sterilization will NOT kill off copepods and other small life forms in a Reef or Nano Reef Aquarium.
This is one of the more laughable myths about the use of UV Sterilizers in reef aquariums as these copepods live at or near the bottom of live rock piles (making a pile with small pieces is best for copepods), they are not active in the water column. If properly installed, the UV should have at least a fine pore sponge filter media as a pre-filter, which will further stop the “ingestion” of these and other minute life forms (the UV benefits as well by being more efficient).
What is interesting about this myth is that many who spread this misinformation use filters such as the Ocean Clear Micron Filters systems (which are excellent micron filters), these filters will filter out any copepods that get caught up in the water column and “sucked” into the filter. As well even “pods” that do manage to find their way into the UV Sterilizer are rarely killed due to size as the typical flow rate of 20+ gph is not low enough to kill them (you would need at least 10 gph per watt, which I do recommend running a UV Sterilizer at flow rate of under 10 gph per watt for this reason).

The bottom line here is that I have maintained MANY Reef aquariums with UVs with growing copepod, anemone and other creature populations. Honestly this is one of the worst urban myths in the aquarium hobby about UVs (mostly spread on the internet in misinformed forums which never conduct or read scientific evidence to back up these absurd statements). The only truth to these statements is that UV Sterilizers can destroy some microscopic food sources needed by some of these organisms (usually planktonic algae, although timers that turn the UV on during certain hours is an easy remedy for this possible problem).

For more on this myth/controversy, please see this article:
More UV Sterilizer Questions/Myths Answered

[6] UV Sterilization NOT remove minerals from aquarium water, however UVC Sterilization will also aid in the removal of oil based (carbon based) pollutants.

[7] The use of Ultraviolet Sterilizers will NOT lower fish immunity, in fact from my many controlled studies, the opposite is true. Although the exact mode is theoretical, evidence points towards improved Aquarium Redox being at least part of the reason.
See also this article: “Fish Immune System and UV Sterilization”

[8] UV Sterilization will NOT make up for poor aquarium maintenance practices such as over crowding, over feeding, inadequate filtration, poor cleaning practices, improper water parameters, and more.
This point is likely the cause for anecdotal statements that fish coming from tanks that had UV Sterilization, then are placed in a tank without an Ultraviolet Sterilizer resulting in “losses” may be dealing with (besides the above point of improved immunity, which will be lowered after transfer). Often an aquarist (I have also performed this as well for studies) will rely too much on the Sterilizer/Clarifier for water quality, clarity, etc., as a UV Sterilizer will often keep a tank clear and healthy in appearance even when correct water changes, mineralization, etc. are not performed. For this reason the fish may not be has healthy as they should since a Sterilizer should NEVER be relied on as a replacement for good aquarium maintenance practices. Along this same line of thought, often aquariums maintained this way will have low KH and falling pH which can result in osmotic shock and even death when transferred.


IN DEPTH INFORMATION ABOUT UV
THAT FURTHER EXPLAINS THE PROPERTIES OF UVC STERILIZATION:

ultraviolet radiation, uv DNA Mutation, light, sterilization
All gases, liquids, and solids are made up of elements. The fundamental building blocks of elements are atoms, which in turn are made of electrons, neutrons and protons..., all held together by electronic attraction. This is referred to as polarity, the principle that positive and negative poles attract and remain bound together based upon the strength of that attraction. There are over 100 elements known in our universe. It is the elements that form compounds.

Elements combine to form gases, liquids or solids. For example, water is made of two molecules of hydrogen and one molecule of oxygen. Carbon dioxide is one molecule of carbon and two molecules of oxygen. These and all other combinations of elements are bound together by the force of attraction or polarity at the level of the atoms. Organic compounds. The compounds of our focus are those structures that are organic in nature. Primarily it is the organic molecules that are the basis of indoor contamination. We need to understand these compounds so we know how to clean and purify the home. Organic compounds are carbon based. Life is determined by carbon based DNA and amino acid chains. Carbon is not only found in "life," but a whole range of chemicals. A number of useful organic compounds are made up of carbon, nitrogen, hydrogen, oxygen and traces of other elements. The most recognizable organic compound is based upon the carbon and hydrogen combination, or hydrocarbons. Plastics, petroleum products and gasoline are hydrocarbons. Hydrocarbon Organic compounds tend to break down or decay faster than non-organic. The decaying process means hydrogen and carbon molecules separate. For example, if the plastic (organic) in milk bottle is left in the sun for a couple of years, much of it will decay. Skin, hair, tissue (all organic) decays more rapidly. This also shows the effectiveness in UV Sterilization in removing oil based pollutants from an aquarium

With the understanding that carbon is the building block of life (and more), we now need to look at the forces that will break down organic and carbon based contaminate molecules. In short, a photochemical process, initiated by short-wave ultraviolet can do this. Short-Wave Ultraviolet we all accept but don’t understand the damaging effects of x-ray and gamma ray radiation. Why isn’t visible light as destructive on human cells or bacteria as x-ray and short wave UV have been shown to be? X-ray, gamma, ultraviolet, infrared and visible light energy all fit in a category called "electromagnetic" energy. They all have the same characteristic "lazy S" energy wave, that travel at the speed of light. The light ray energy is called photons that oscillate, resulting in wave frequency. The difference in each type of wave energy is the wavelength, the distance across this wave. By definition, the shorter the distance across the wave, the more powerful the wave will be. The difference in the wavelength determines how the wave affects its surroundings. It is this wavelength difference that allows short-wave x-ray to pass through walls, while longer-wave visible light cannot pass though the same material; short-wave ultraviolet and x-ray can destroy DNA in living microorganisms and breakdown organic material while visible light will not.

Nanometers: Measuring Light Energy All light energy is measured on a "nanometer" (nm) scale. Nanometer means one-billionth of a meter. The lower end of the scale has the shortest wavelength, and the upper the longest. Cosmic, gamma, x-rays and "C" band UV are all classified short-wave energy. Visible light is at middle ground, at 400-700 NM on the scale. Infrared light is in the upper end of the spectrum, running from about 800 to 1400 NM, and radio waves are longer yet in the 1400 to 2200 NM range.
Ultraviolet light is toward the low end of this scale, from about 100 to 400 NM, with three categories, "A," "B" and "C." UV is beyond the range of visible light and cannot be seen. We only see evidence of its presence. Short-wave UV, called "C" band (100 – 280 NM) is known as UVC. Most C band radiation is screened from the sun before reaching the earth by the production of ozone in the upper atmosphere. Useful UVC is entirely manmade, found in today’s low-pressure UVC lamps. The most effective sterilizing range for UV is within the C bandwidth. This range is called the germicidal bandwidth. The ideal germicidal curve is considered 240 NM to 280 NM, with the most effective at 265 NM. With the initial exposure, UVC has properties that alter the cells of living tissue, particularly microbes. UVC radiation triggers the formation of peptide bonds between certain amino acids in the microbe’s DNA molecules. This renders bacteria, viruses and molds harmless by robbing them of the ability to reproduce. Most research points to a microwaves per second by cubic centimeter (mW/cm2) of 4,200 to 8,700 to destroy bacteria of their ability to reproduce. (please see the next section for more about mW/cm2)
If the germ cells are exposed for longer periods, they start breaking down to the molecular level (carbon, oxygen, hydrogen, nitrogen ions, etc.).

UVC Germicidal Effectiveness Wave length; NM Relative germicidal effectiveness
240 .62
245 .72
250 .90
255 1.03
260 1.12
265 1.15
270 1.08
257 .98
280 .87
285 .73
290 .60

The optimal wavelength for germicidal effectiveness is shown to be 265 NM


Aquarium & Pond UV Sterilization Summary

This article is quite in depth, especially when one reads all the outside references/sources.
However if one conclusion/summation can be honestly be made by reading this article and the resources in their entirety, that is that although one CAN maintain a healthy aquarium or pond without a UV Sterilizer, adding this device greatly increases your chances of success and when one considers the price of many UV Sterilizers vs. the cost of replacing valuable fish, not to mention time, possible remedies, and heart ache it is often foolish to dismiss a UV Sterilizer as an irrelevant piece of equipment OTHER USES FOR UVC:

UVC Sterilization is not just limited to Aquarium and pond use. The use of UVC Sterilization is growing in popularity for use in foods and food preparation. UVC is also used for Indoor air quality as a means of purifying the air that we breath, removing harmful pathogens from the air.

Resources:
*http://www.uv-disinfection.com/html/english/glossary/index.htm
*http://www.ccalmr.ogi.edu/~kilgren/mtu_senior_design/MTU_task14_senior_design_2003_report.pdf
* http://www.lns.cornell.edu/spr/2004-09/msg0063557.html
*http://rael.berkeley.edu/old-site/uvtube/uvdisinfection.htm
*http://www.hygienitech.com/Hygienitech%20UV-C%20Light%20Primer.pdf



HERE IS AN INTERESTING ARTICLE AND QUOTE:
(I am posting this here to make readers aware that although UV Sterilizers are not essential, reasons such as this are poor ones to not have a UV).

I was going thru Yahoo Answers, as I was curious about this feature.
I stumbled upon this gem;

Question:
What is the best UV Sterilizer for use in a 20 gallon tank and where can I order one online?

Answer:
A UV sterilizer might do more harm than good to a 20 gallon tank. The UV sterilizer will do exactly that; sterilize almost everything that passes through it, bad bacteria and the good. In a 20 gallon tank your water will circulate so many times that you may be doing more harm than good. There are other ways of treating the water in your tank that might not be has extreme as a UV sterilizer.

TOTALLY AND COMPLETELY WRONG!

This a good example where caution needs to exercised not just on the internet, but at many so-called aquarium stores staffed by un-knowledgeable aquarists. This person probably has never even used one to make such an outrageous statement such as this.
Often many aquatic forums are staffed by after work aquarium “experts” who have not done their homework or worse, they are just plain dishonest. (This is not to say there are not a lot of good ones out there, but I have left a few after being flamed there by aquarists who knew nothing about which they were talking about).
I have spent 25 years studying, reading, and doing tank by tank comparisons in my aquarium maintenance business. A UV should be properly installed such as not to high or slow a flow rate. Also the watts per gallon have to be figured.
Another often missed aspect is the Redox Potential which research shows having a proper Redox Potential around 300 mV improves the water quality much the same a way an anti-oxidant vitamin works. A UV sterilizer properly installed helps with this!
Also in studies with goldfish (a very dirty fish), I have found vastly healthier fish in aquariums with UVs vs. without, all other aspects such as feeding and filtration equal.

Another incorrect point was about the “good and bad” bacteria. Aeromonas bacteria are common in most all aquariums and pond they are only “bad” when they become pathogenic due to an injured or weak fish, or very common, when water conditions are poor. “Good” bacteria such aerobic nitrifying bacteria can be “bad” when there is a bacterial bloom, as they rob oxygen from the fish, weakening fi