What is the nitrogen cycle and how does it effect keeping fish? What is 'cycling' a tank and how is it done?


1 Answer 1


The nitrogen cycle is the cycle that waste products go through in a fish environment - this happens in both small aquariums, to huge tanks and ponds - and in the wild as well.

This process can take from 2 weeks to a number of months pending on various factors such as the size of the tank and the number of fish you wish to keep.

It is vital for anyone planning on keeping aquarium fish to understand this process and will help you to be successful in keeping fish. The best way to monitor the nitrogen cycle is to purchase an aquarium test kit that will test for ammonia, nitrites and nitrates.

Nitrogen Cycle Stages

Stage 1

Ammonia is introduced into the aquarium via fish waste and uneaten food. The fish waste and excess food will break down into either ammonium (NH4+) or ammonia (NH3). Ammonium is not harmful to fish but ammonia is. Whether the material turns into ammonium or ammonia depends on the pH level of the water. If the pH is under 7, you will have ammonium.

Stage 2

Soon, bacteria called Nitrosomonas will develop and they will oxidize the ammonia in the tank, essentially eliminating it. This is the point at which you may start seeing a grey/green bloom in your tank water. The byproduct of ammonia oxidation is nitrites. Instead of ammonia in the tank, we now have another toxin to deal with - nitrites. Nitrites are just as toxic to fish as ammonia. If you have a test kit, you should be able to see the nitrite levels start to rise around the end of the first week.

Stage 3

Bacteria called Nitrobacter will develop and they will convert the nitrites into nitrates. Nitrates are not as harmful to fish as ammonia or nitrites, but nitrate is still harmful in large amounts. The quickest way to rid your aquarium of nitrates is to perform partial water changes. At first this should be as high as 50% changes, but once your tank is established it can be lowered, usually to around 30% weekly but it varies by tank and number of fish.

Fishless Cycling / Additives

While the most well known method of cycling a tank is adding a bit of fish food and allowing it to produce ammonia to start the tank cycling, this can be prone to failure for a number of reasons - if the bacteria aren't 'fed' enough or too much then you can end up with a nasty looking tank and still no where safe for fish.

Because of this there have been various products created which will help speed the process along, or in some cases 'replace' the bacteria which process the nitrites with others that come in a bottle form. If using these you should always follow the instructions to the letter.


Fishlore - Nitrogen Process

Fish Nitrogen Cycle

The Fish Wiki - Nitrogen Cycle

  • 1
    Do you have any links with that? (sound right, but a good source is always nice to have!) Commented Jan 2, 2018 at 15:42
  • 1
    @Flummox yup will be links, still working on the content too. Commented Jan 2, 2018 at 15:43

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