The Fluval Poly/Clearmx filter cartridge helps to control green water and unsightly algae growth by adsorbing and trapping phosphate, nitrite and nitrate. The filter pad provides crystal clear water, a reduction of odors, creates ideal conditions for fish and plants in an aquarium and helps to reduce maintainances. Replace the Fluval Poly/Clearmax filter insert every 6 to 8 weeks or when flow rate is significantly reduced. To ensure constant presence of beneficial bacteria, replace media cartridges one at a time and never together..for Fluval Internal Filter U2, Fluval No.: A481, content: 2 piecesfor Fluval Internal Filter U3, Fluval No.: A482, content: 2 piecesfor Fluval Internal Filter U4, Fluval No.: A483, content: 2 pieces