Bacto-Surge ® Biological Action Filters are air-driven sponge filters (air pump not included) which offer superior mechanical and biological filtration of your aquarium water. The highly porous sponge contains extreme surface area and thousands of microscopic pores where aerobic bacteria, hydrated with oxygenated water, can grow and flourish. Great For Use In Freshwater Or Marine Applications Highly porous sponge maximizes bacterial action Increased surface area offers maximum bacteria colonization Soft sponge material is easily cleaned Polyether foam for longer life Uplift tube and air diffuser combination provide maximum lift and won't easily clog Other uses: polishing filter for canister filter applications, hang on filter applications or uplift tubes of undergravel filters. Sizes Mini - #96038 (For use in aquariums up to 10 gallons) Small - #96040 (For use in aquariums up to 40 gallons) Large - #96044 (For use in aquariums up to 75 gallons) X-Large - #96048 (For use in aquariums up to 125 gallons) Note: The finer hole structure allows the Bacto-Surge ® Foam Filters to provide significantly more bacterial colonization than competitive foam filters of similar size.