The Nitrogen Cycle in Aquariums

What is the nitrogen cycle?

Understanding the nitrogen cycle is the most important stage in keeping healthy fish. When a new tank is first started up, we need to establish the nitrogen cycle to ensure toxins from fish waste are cleaned from the water and so cannot harm the fish. This means establishing a nitrogen cycle – or cycling the tank.

Simply put, fish excrete ammonia from their gills and as waste from their digestion. Solid waste also breaks down to form ammonia as does leftover food, decaying plant matter and so on. Ammonia is very toxic to fish so we need a way to remove it from the water to protect them from its effects. The filters in our tanks don’t in themselves do anything to deal with ammonia: they remove floating particles as the water circulates through them and they add flow and oxygen which are good for the fish, but a brand new filter won’t do anything about ammonia.

Biological Filtration

Filters contain sponges or ceramic balls or similar and water which contains ammonia attracts beneficial bacteria whose food source is ammonia. They will colonise the filter media and supplied with flowing oxygenated water passing through the filter, will extract ammonia from it and use it as food. As they do so, they produce their own waste product; nitrite. Unfortunately, nitrite is also very toxic to fish, but once nitrite starts to accumulate in the water, a second type of bacteria will also colonise the filter media and use the nitrite as food. These bacteria excrete nitrate, which is far less toxic than either of its precursors and won’t harm fish unless it’s allowed to rise to high levels. Nitrate is removed from the water by changing some of the water every week and replacing it with fresh water.

Because ammonia, nitrite and nitrate are all forms of nitrogen, we call this the nitrogen cycle.


Illustration of the Nitrogen Cycle

It takes quite a long time for the ammonia-eating bacteria to arrive and grow a big enough colony to deal with the ammonia in a new tank which leaves fish exposed to harm from increasingly toxic levels of ammonia for several weeks. Once ammonia starts being processed it takes another few weeks for the second type of bacteria to arrive and start processing nitrite, leaving fish exposed to high nitrite levels for that period. For these reasons, it’s no longer considered acceptable to use fish to cycle a tank and the correct way to do so is to fishless cycle, using a non-living source of ammonia instead.

Of course, many new fish keepers aren’t aware of fishless cycling and some still, unfortunately, find themselves with an uncycled tank having been poorly advised and so have to cycle with fish. If that has happened to you, you can reduce the fishes’ suffering and prevent their deaths from ammonia and nitrite poisoning by following this guide to dealing with ammonia and nitrite spikes.

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