Aquatop Power Filter The AquaTop Hang on Back Fish Tank Power Filter for Tanks up To 30 Gallons Power Filter by Aquatop, is an easy to use 3 stage hang on filter that features mechanical, chemical, and biological filtration. The AquaTop Hang on Back Fish Tank Power Filter for Tanks Up To 30 Gallons, with Flow Rate uses an easy to change drop in Filter cotton pouch that is filled with activated carbon to provide you with your mechanical and chemical filtration. for you biological filtration, the AquaTop Hang on Back Fish Tank Power Filter for Tanks up To 30 Gallons has a unique “Bio-Wall” insert that allows for superior biological filtration. Efficiently removing toxic ammonia and nitrite. As water first passes through filter, it reaches the unique “Bio-Wall”. This is where the beneficial bacteria that is colonizing on the wall will break down the toxic ammonia and nitrite. Next, before the water returns back into the aquarium. It passes through the cotton wrapped carbon insert. Here it goes through the mechanical and chemical filtration process. Allowing the carbon to remove discoloration, odors, and other impurities from your water.