The sponge must be cleaned regularly, before the solid waste it filters out decomposes and is released back into the tank. Some pet stores carry specialty power filters, which either add cleansing chemicals to the water or filter the water over a concentrated region of cleansing bacteria. Consult a pet store before purchasing one of these. Chemical Filtration Chemical filtration uses gas-activated carbon to remove dissolved materials. It is a highly effective way to clean an aquarium.
Gas activated carbon is cheap, and is easily spread into a tank.
The best gas-activated carbon is made from coal, and absorbs waste through a process called molecular sieving. Be sure to wash the carbon free of contaminants and silt before using it.