3 different replacement filter cartridges each designed for the Fluval U2 filter. The U2 Clearmax Cartridge helps control green water and unsightly algae growth by trapping and adsorbing phosphate, nitrite and nitrate. the U2 Bio-Foam Pad captures large particles and debris for effective mechanical filtration. The U2 Poly-Carb Cartridge has two unique filtering sides that combine to provide thorough chemical filtration.