Keep your Fluval U4 Underwater Filter working effectively and help to control green water and algae growth with Fluval U4 Poly/Clearmax Filter Cartridge. Fish waste, uneaten food and other organic matter can build up in your aquarium water, causing algae growth and increased phosphate, nitrite and nitrate levels. Raise these levels too much, and they can be harmful to fish. This filter cartridge helps to absorb and trap these pollutants, protecting your aquarium and those who call it home. Designed for the Fluval U4 Underwater Filter, Fluval U4 Poly/Clearmax Filter Cartridge supports crystal clear water, helps to control odor and works to reduce overall aquarium maintenance. This is also great for use with aquatic turtles and terrestrial turtles.