Chemical filtration removes microscopic impurities from your aquariums water through a chemical reaction. Activated carbon is the most common filter media used for chemical filtration however there are plenty of others such Seachems Purigen, Macropore Gold and Aquaholics very own nirite pads and phosphate pads just to name a few! Microscopic impurities or harmful bacteria can become a dire issue for fish tanks even with a great filtration system, Chemical Filtarion deals with this this by removing and filtering these micorscopic particles.