Each of these inserts are designed specifically for the Fluval U3 filter, the U3 Poly-Carb Cartridge has two unique filtering sides that combine to provide thorough chemical filtration. The U3 Bio-Foam Pad captures large particles and debris for effective mechanical filtration. The U3 Poly-Max Cartridge helps control green water and unsightly algae growth by trapping and adsorbing phosphate, nitrite and nitrate.