Under Gravel Filter 7.8"x5.5" Undergravel Filteration for Fish Tank Air Pump Caring Guide: At least 1~2 inches of washed gravel should be placed on top of the filter plate before water is added. The Flow Under gravel Filter is a biological filtration system for fresh or saltwater aquariums. The filter circulates water through the gravel bed and oxygenates it for beneficial bacteria growth. Pls see the Pipe diameter on item page, it's the size suitable with a filter, and it could using with any kind of filter, or a submersible pump or outer filter. Under Gravel Filter x1 (19.9x14cm / 7.8"x5.5")