Fluval "U" Poly/Carbon Cartridge 2Pack. Fluval Poly/Carbon Cartridge has two unique filtering 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. Key Features : Polyester/Carbon cartridge provides thorough and effective chemical filtration Polyester filter pad side filters out fine debris and contaminants Carbon side filters out harmful liquid substances, colourants and odours For freshwater, marine or reptile environments Contains 2 Poly/Carbon cartridges Available Sizes: "U2" : For Fluval U2 Underwater Filter "U3" : For the Fluval U3 Underwater Filter "U4" : For the Fluval U4 Underwater Filter.