The Fluval Poly/Carbon Cartridge has two unique filtration sides that combine to provide thorough and efficient chemical filtration, the crucial second step in the filtration process. The side with the polyester filter pad filters out fine dirt and impurities. The carbon side filters out harmful liquid substances, dyes and odors. Key features: Polyester/carbon cartridge provides thorough and effective chemical filtration. The polyester filter pad side filters out fine dirt and pollutants The carbon side filters out harmful liquids, dyes and odors For freshwater, marine or reptile environments Contains 2 poly/carbon cartridges for Fluval Underwater Filter