Your filtration system should incorporate all three. Mechanical Filtration Mechanical filtration is the means by which large particles of excess food and other debris are removed, screened, or skimmed from the water. This is achieved by flowing water through fiber floss, gravel, foam, or some other screening material. While mechanical filtration uses filters to remove debris, chemical filtration uses activated carbon and ammonia absorbents, such as zeolite, to remove odor, colors and harmful substances, such as ammonia, from the water. With activated carbon, each piece of carbon is like a little sponge that traps odors, medication residue, and dissolved fish waste.
Carbon also removes discoloration in cloudy water.
Activated carbon will no longer effectively absorb anything and should be replaced after about 3 or 4 weeks. To test the absorption power of carbon, take a glass of aquarium water on white paper. If the paper looks yellow through the glass, it's time to change the carbon. Ammonia can be removed by using a liquid ammonia remover that is placed directly in the water, or ammonia chips, which are put into the filter. The primary means of removing ammonia and nitrites should always be your biological filter.