For use in freshwater or saltwater aquariums. Carbon cartridges and air diffusers included. Biological filtration is the process of breaking down toxic waste material to less toxic nitrates by beneficial bacteria. Colonies of bacteria present on the surface of your aquarium, thrive on a constant supply of oxygen and media (gravel) with large amounts of surface area. Lees Under Gravel Filter is the key to providing a strong biological filtration system. Placed under the gravel on the bottom surface of the aquarium, the filter plates utilize the gravel bed as a source area and provide oxygen by recirculating the water down through the media and up through the uplift tubes