Dimart XY-1038 Aquarium Fish Tank Reusable Black Filter Biochemical Cotton Filter Foam Sponge for Fish Tank Pond The Biochemical Filter has the best effect on decomposing the excrement and the forage leftover. It can be used in sea water tank, fresh water tank, aqua-plant tank. Thickening biochemical filter sponge, tough and durable. Good shading features, can reduce the algae breeding. Moderately hole, can cultivate high density nitrifying bacteria. Strong filterability, can stable water quality and reduce fish died. Can be used repeatedly after cleaning. Advise to clean it each month.