Activated Carbon works both by the process of absorption and adsorption. Absorption is the process by which pollutants or impurities in water are trapped in the molecular pores of the activated carbon and adsorption is the process by which pollutants are linked or bonded to the surface of the activated carbon thereby completely removing from the water medium. When activated carbon is used as filtration medium it will take in all the pollutants or impurities from the water and helps to improve its quality.
Activated carbon can also help in keeping the nitrate level in water under control by its biofiltration properties. Nitrates in excess level is poisonous to fishes and is the main reason why water changes are required in the aquarium. Nitrates are end products of nitrogen cycle in aquariums and hence need to be removed.