The Nitrogen Cycle

Alistair Gamblin, 23/02/2026

What is it? 

All fish excrete ammonia as their waste product. In its raw form this is highly toxic, and if it is not removed effectively from the water, it can reach dangerous and even fatal levels. Thankfully, the natural process of the nitrogen cycle comes into play, and neutralises harmful waste into something much safer, even helpful to other aquatic life.  

The first stage of the nitrogen cycle is the production of waste. This could be from decaying matter, such as plants, excess food, or fish waste. As this waste decomposes in the aquarium, ammonia is produced. Simply, this waste then goes on to be utilised by other organisms- such as plants- in the ecosystem, and the cycle continues as the fish then consume these. There is, though, a slightly more complex process behind all of this, which takes place over several stages. 

Once the waste has been produced, the aquariums beneficial bacteria get to work. Firstly, the ammonia is broken down by a bacteria known as Nitrosomonas. The resulting product is called nitrite. In this form, the waste is still slightly toxic, but less harmful than ammonia. 

Next, another type of beneficial bacteria gets to work: Nitrobacter converts nitrite into nitrate, which is again a much safer form of fish waste. The bacterial process is shown simply in the diagram below- the colours indicate how each time waste is converted, it becomes less toxic. Only small levels of ammonia and nitrite in the water can be greatly harmful to the aquatic life in the system, whilst nitrates can build to much higher levels without ill effects.

Still, the process is not quite over, and the levels of nitrate still need to be managed. In nature, live plants will then absorb this final product and use it for nutrition. Then, the cycle starts all over again as aquatic animals eat and digest plant life and excrete the waste product. 

Why is it important, and how does it work in my aquarium? 

In a home aquarium, the nitrogen cycle forms an important part of filtration, maintaining the high water quality needed by your fish and other inhabitants to survive and thrive. It is known as biological filtration (i.e. a biological process through which harmful compounds are removed) and is one of the 3 key methods of maintaining water quality. The others are: 

• Mechanical filtration- this is the physical removal of waste particles. In a fish tank, this is often achieved by using sponges and screens in the filtration system 

• Chemical filtration- this is the use of chemical products to reduce harmful substances. Items like activated carbon are often used in home systems, whilst commercial and public aquaria may utilise ozone 

A combination of each of these methods is important in creating a balanced and effective system. In fact, the biological process of the nitrogen cycle can be reliant on working alongside other methods. For example, the beneficial Nitrosomonas and Nitrobacter bacteria need places to live within the aquarium. Substrate and filter sponges provide the perfect climates for these bacteria to thrive and carry out their important work, and some- usually larger- systems will also have dedicated biological media. These are usually made of materials such as ceramic or plastic and are designed with a large surface area to host healthy bacteria populations. 

It is for this same reason that cycling your aquarium is required before introducing livestock. The process of cycling an aquarium gives the system time to settle and allows for the growth of the beneficial bacteria required for the nitrogen cycle to begin. Most aquariums will require at least a couple of weeks of cycling before fish can be safely introduced.

The above diagram gives a very simplified overview of the stages within the natural nitrogen cycle. As home aquariums are much smaller than the wild, the process can be slightly different, and fish keepers can struggle to keep nitrate concentration to a safe level without additional measures. Whilst a naturally planted tank will absorb a degree of nitrate, often additional water changes are required. The frequency and volume of these changes will be dependent on aquarium size, stocking density, and several other factors. Fish keepers should use water test kits to determine the levels of ammonia, nitrite and nitrate in their systems, allowing them to carry out the appropriate remedial action as a result.

In summary...

The process of the nitrogen cycle is vital for the success of a healthy aquatic system, both in the wild and in home and public aquaria. All fish keepers should have a basic understanding of how this process works and why it is important, and take steps as a result to ensure that their fish remain healthy in their environment.