The Fluval Poly/Carbon Cartridge has two sides that combine to provide thorough and effective chemical filtration, the crucial second stage in the filtration process. The polyester filter pad side filters out fine debris and contaminants. The carbon side filters out harmful liquid substances, colourants and odours. For freshwater, marine or reptile environments. Includes 2 Poly/Carbon cartridges. For the Fluval U4 Underwater Filter.