In an enclosed environment such as a home aquarium, waste (feces, uneaten food and plant matter) accumulates rapidly. For this reason it is of utmost importance to have a strong biological system to break down such waste material before it becomes toxic to the aquarium inhabitants. Biological filtration is the process by which waste is broken down from toxic ammonia to less toxic nitrates (see illustration of nitrogen cycle) by beneficial bacteria that inhabit the surfaces in the aquarium. The optimum conditions required for the bacteria to prosper are a constant supply of oxygen and media with large amounts of surface area for the good bacterial to colonize. The undergravel filter allows you to utilize the gravel bed as a source of surface area and provides large amounts of oxygen by recirculating the water down through the gravel and up the uplift tubes, creating a constant flow of water through the media. Biological filtration may also be accomplished with LEE'S Bio-Pin Balls or Sponge Filters.