The Nitrogen Cycle is probably the most important process occurring in any aquarium or pond. Failure to appreciate its action will lead to unsuccessful fishkeeping in the long term.

Basically, the nitrogen cycle is Nature's 'waste management' programme. Wastes, which would otherwise pollute the water and present problems for the fishes' health, are converted to harmless products in a circular process, hence the name.

However, in most aquariums only half the process occurs, as most filtration systems do not have a specially-designed processing unit to deal with nitrate - the end product of nitrifying bacterial activity.

As can be seen from the above diagram, all wastes begin with ammonia but are converted into nitrite by Nitrosomonas bacteria living in the biological filter's medium. Next, nitrite is converted to nitrate by a further bacteria, Nitrobacter.

If the amounts of ammonia, nitrite and nitrate are monitored during the establishment on a new aquarium then the peaks should follow the above pattern over the course of a few weeks.

It is vital that the biological filter bed is established before any livestock is added, however, this poses a problem as the bacteria won’t get established until they get some ‘waste’ to work on! Fortunately, you can use several methods to give the bacteria a ‘kick-start’: add some gravel from an established aquarium into the new one; add a little food and let it decay in the water; add any of the commercially-available ‘starter cultures. Above all, when fish are added, don’t be tempted to load the tank up to its theoretical maximum straightaway - let the filter keep in step with the increasing load.

Nitrates are utilised by aquarium plants and this, backed up by regular partial water changes should keep nitrates down to a safe level. Excessive nitrates (together with phosphates) will encourage algae. The removal of nitrates is also possible by using resins (preferably to treat water before it’s used in the aquarium water, and certainly when used for marine tanks).

‘De-nitrating’ filters are sometimes seen but these have to be managed very carefully, the bacteria in them require very low oxygen levels and often need feeding. The use of Living Rock in marine aquariums offers a complete nitrogen cycle process: bacteria on the external surfaces in oxgenated water deal with ammonia conversion to nitrite and nitrate, whilst the action of bacteria deep within the porous rocks then reduces nitrates.

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