- Description Fluval Poly/Clearmax filter cartridge helps control green water and unsightly algae growth by adsorbing and trapping phosphate, nitrite and nitrate. It effectively produces crystal clear water, reduces odors, and creates ideal conditions for fish and plants. The filter cartridge helps reduce overall aquarium maintenance. It is ideal for freshwater and marine environments and designed for Fluval’s U4 Underwater Filter. Contains 2 cartridges.