Offering outstanding 3-stage filtration, increased water movement and vital aeration, Fluvals U-Series internal filters are the perfect solution where external filtration is not possible.This 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, while the carbon side filters out harmful liquid substances, colourants and odours.