What is Chemical Filtration?

Chemical filtration is one of the three types of filtration that can be used for an aquarium. The other two are biological and mechanical filtration.
Biological filtration removes ammonia and nitrite from aquarium water by way of positive bacterial colonies that convert it, through oxidation, to harmless nitrate.
Mechanical filtration is performed when water passes through a filter's foam pads, catching particulate matter like uneaten food, waste and decayed plants.
Mechanical and biological filtration are the basics of a healthy aquarium, while chemical filtration is an option.
Chemical filtration refers to any filtering substance that is designed to change the chemical composition of the water, but most often refers to the use of activated carbon or other cleaning resins. Activated carbon pulls dissolved organics from the water by adsorbing them.
Activated carbon is made from various base materials that have been heated then steam-treated. The steaming process makes the carbon extremely porous. Porous substances have extensive surface area. As the water passes over carbon, the carbon chemically attracts pollutants that adhere to the surface of the carbon, including odors and color. This is chemical filtration in action.
Note that carbon with excessive ash can cause a pH spike. Therefore when selecting carbon, acid-washed carbon is best. When you purchase high grade carbon, the package will usually claim that it will not alter pH. In addition to these considerations, spherical-shaped carbon is superior because it does not pack and has the highest surface area, allowing the greatest adsorption.
Since carbon works by chemically attracting pollutants to its surface, once the surface area is covered with dissolved organics, the carbon is exhausted and must be replaced. Depending on many factors, this can happen anywhere from a few days to 3-4 weeks. One signal that your carbon is exhausted will be a gold tint to the water.
Because standard activated carbon exhausts so quickly, many aquarists turn to specialty resins that include high grade carbon along with a proprietary blend of other compounds to provide superior chemical filtration. One such product is BioChemZorb by Aquarium Pharmaceuticals. BioChemZorb comes in a sealed filter bag, ready to rinse and place in a filter. It normally lasts about 6 months. BioChemZorb not only cleans water but also polishes it.
There are also other products that fall under the heading of chemical filtration. Products like ammonia chips, designed to remove ammonia from the water. People who have excess nitrates might use ammonia chips to slow nitrate production. Nitrates are not considered harmful in general, but an excess of nitrates is undesirable, particularly in salt water aquariums where nitrates must be kept very low.
Whenever considering a product for the first time, make sure to read the label carefully. Some products alter pH inadvertently, some might alter hardness. Others might add phosphates, or have some other undesired effect. Some products are meant for freshwater only, others for salt water. When you find the product you want, make sure it states clearly that it will not change the water except in the ways you intend.