The Nitrogen Cycle

Care, Freshwater, Plants, Shrimp -

The Nitrogen Cycle

The nitrogen cycle is the one most important and often least-known subject for aquarium keepers. As long as you understand how this works in your aquarium however, keeping fish can become super easy and stress-free. 

The Nitrogen cycle describes the main biological process which occurs in every tank (both freshwater and salt-water). In one sentence it is: Fish create Ammonia from their waste and rotting food in the tank (deadly to fish and shrimp); then bacteria in the filter convert it to Nitrites (still deadly) and more then convert it to Nitrates (largely harmless) which is what you remove through water-changes. 

Yes, that's it!

Once you really understand this though, there are a few consequences and best-practice that you should keep in mind when setting up or maintaining your aquarium. We will go over them below.

You need a filter.

A lot of new keepers are often daunted by the supposed maintenance that aquariums need, leading to the very popular idea of "Filter-less" and "No water change" setups. To be clear, these are possible to achieve, but for your first tank I would strongly recommend against it. The crucial thing a filter supplies is water-flow: Just like your lungs need you to pull air in and out of them to get enough oxygen, so too do the bacteria in the filter need running water to be able to pull enough chemicals from it. A real-life example of this is streams and ponds: Ever wondered why streams are often crystal clear while stagnant pools are not? 

A filter does not add maintenance to your tank either (if anything it reduces it) so in almost every case; you should get a filter. (ideally 6-10x your tank size in volume per hour. So a 50L tank should have at least a 300L/h filter)

Run your filter for 1-2 weeks before putting in livestock.

So, there is bacteria in my filter... Where does this bacteria come from?

Luckily for you, you don't need to get any extra products for this (although it can help), all the bacteria you need are already everywhere around us if only in very small quantities. But what you do need is for the bacteria you want to increase in quantity enough so it can sustain any livestock you put into the tank from the first day!

It takes about 1-2 weeks for this to happen in a ideal setup, and this is what we call "cycling your aquarium"; running your setup like normal for a while before adding any livestock. Here are a few things to keep in mind to make sure this happens properly:

  • You need to actually have Ammonia in the tank to start the cycle:
    Add a little fish food into your tank, or if you have aqua-soil that is perfect. You can also consider adding a small hardy fish or shrimp in sooner if your tank is large enough.
  • Don't fill your tank with fish all at once. Add your livestock gradually over a week or more to let the filter adjust.
  • Add plants immediately. Plants have plenty of bacteria on them themselves, while also affecting the cycle. Adding them from the start is no problem.

Don't have too many fish

A lot of problems people seem to have often stem from them just having too many fish in the tank for its size and/or filter capacity. A good rule of thumb is 1cm of fish for 2 Litres of water but the lower this number the less problems you will have.

Start with a larger tank

Counter-intuitively, larger tanks are actually easier for first-time fish keepers. This is because stability is important, and a large tank with more water is much more stable than a small one.


Nothing in your aquarium will effectively remove Nitrates. Although they are harmless, everything stops being harmless in large enough quantities. A water change of 20% per week is enough to deal with this, and aquatic plants will always help!


This should be all the most important things you need to know to keep fish. Shrimp and snails all follow the same rules, only shrimp are a little more sensitive to ammonia, while snails less so. 


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