In still waters like a bucket, this process is not very quick. Aeration in aquaria is designed to increase this gas transfer. The best way to do this is to increase the surfaces by which oxygen can enter the water, which is done by increasing the turbulence at the water surface. This can be achieved in lightly stocked aquaria by the use of the filtration equipment which circulates the water, and can agitate the water surface by a variety of methods including spray bars. Biological Filtration Wastes are expelled directly into the water in the forms of organic waste, urea, ammonia etc.
Ammonia is excreted by fish and invertebrates and results from the break down of organic matter.
Ammonia is toxic to fish in amounts too small to be accurately tested by the home aquarist. In conventional marine aquaria the answer to this problem in bacteriological control whereby a suitable medium is provided to provide a home for helpful aerobic bacteria called nitrifiers which convert the toxic ammonia into less toxic nitrites and to nitrates. The usual medium is gravel or coral sand which is used in the undergravel system of filtration where a powerhead is used to circulate the aquarium water by drawing it through the gravel. The nitrifying bacteria live amongst the gravel. In the external system, the gravel, or alternative medium, is housed in a canister.