From The Aquarium Wiki
What is Anaerobic
An anaerobic organism or anaerobe is any organism that does not require free-standing oxygen molecules for growth.
Typical organisms in a freshwater aquarium is anaerobic heterotroph bacteria found in the deep layers of the substrate where fresh water can't easily flow and so the water becomes depleted of oxygen and the bacteria takes oxygen from the nitrate molecules in the water (As nitrate molecules are made up of nitrogen and oxygen atoms) thus leaving pure nitrogen. Some heterotroph species of bacteria will even convert nitrogen back into ammonia and nitrite in such anaerobic conditions.
- It has been often reported of aquarists moving a large piece of ornament after a long time and having large quantities of this trapped gas given off and it can cause fish to die fairly quickly if exposed to it.
To reduce the levels of this bacteria, the substrate needs to be kept stirred with a long stick once a week. Or the layer of substrate is kept relatively thin (around 1" thick)
Sand substrates are the worst (or the best depending on your point of view) for promoting this bacteria.
- The opposite is aerobic.
Making use of anaerobes
In marine aquariums this organism is encouraged in order to reduce nitrate levels in the water by having beds of sand and pumping water slowly through it.
A cheap form of nitrate remover for freshwater tanks is easily made by having 20+ feet of 12mm diameter hose coiled up and a pump forcing tank water through it at a very slow rate, less than 1 drop per 5 seconds.
The slow flow of water through the coil causes the bacteria to consume the oxygen and therefore anaerobic bacteria starts to grow eating the nitrate.
After a few weeks this flow can be slightly increased and therefore you have a cheap and effective nitrate remover.
- The water coming out of the coil is however slightly acidic and lacking in oxygen so this should be dripped onto the surface of the tank.