One of the most important things in creating a healthy and safe ecosystem in your aquarium is to understand the nitrogen cycle. Beginners that are unaware of how it works and how to detect which state their thank is in, often make big mistakes that can lead to plants or animals dying in the most extreme case. Once you've made yourself familiar with it, it's very easy to understand and control.

The picture above gives a quick overview of the processes involved in the nitrogen cycle and this article will explain each step in-depth and what to do during spikes.

Please don't leave your aquarium unattended for too long during the cycle phase so that you can react immediately in case something happens.

What is the nitrogen cycle

The nitrogen cycle is a natrual and continuous process where waste is being turned into nutrients with the help of bacteria. In new aquarium setups this cycle first has to be established before it can naturally continue on its own. This procedure is known as "cycling".

It all starts with waste from fish (excrements), excess feeding (fish-food) and plants (rotting plant parts) that sinks down onto the soil / substrate layer of your tank. The various beneficial bacteria in the soil / substrate will then help transform this waste into beneficial nutrients for the plants.

Steps in the nitrogen cycle

The nitrogen cycle has three distinct steps: Waste turns into ammonia, ammonia turns into nitrite and nitrite turns into nitrate.

Each of the steps is explained in detail below:

Step 1: Waste -> Ammonia

No matter what kind of aquarium you have, you will either have waste from fish or plants or both. This waste will start to decay and break down into carbon dioxide (CO²) and ammonia (NH³) mostly by itself.

Carbon dioxide (CO²) helps plants strive and grow faster / bigger. In aquatics some people also like to add carbon dioxide (CO²) through an external source to push the plant growth to the limits. Some plants only develop their special red colors with excessive carbon dioxide (CO²) levels.

Ammonia (NH³) is harmless to plants, but toxic to livestock. This is number one out of two water parameters that need to be checked constantly, especially at the beginning of a new aquarium setup and after thoroughly cleaning the aquarium or filter.

Step 2: Ammonia -> Nitrite

When newly setting up an aquarium there are hardly any beneficial bacteria in your tank. These need to be grown by feeding. The first bacteria required is the nitrosomonas bacteria (beneficial bacteria). These bacteria help to transform ammonia (NH³) into nitrite (NO²). So in order to grow the population of nitrosomonas bacteria you need to add either fish food to decay into ammonia (see step 1) or ammonia itself.

Nitrite (NO²) is harmless to plants, but toxic to livestock. This is number two out of two water parameters that need to be checked constantly, especially at the beginning of a new aquarium setup and after thoroughly cleaning the aquarium or filter.

Step 3: Nitrite -> Nitrate

The other beneficial bacteria required for a nitrogen cycle is the Nitrobacter Bacteria. You can grow the population by adding additional nitrite to feed them, but we do not recommend it. The nitrite level will build up by itself by adding fish food or ammonia as seen in step 2. Nitrobacter Bacteria help to transform nitrite (NO²) into nitrate (NO³).

Nitrate (NO³) is a nutrient which plants require to grow and be healthy. Up to certain levels (below 100 ppm) it is harmless to livestock. But remember that nitrate not only benefits healthy plant growth, but also algae growth. Therefore, it is best to have almost no nitrate in the water parameters as that's an indicator for the plants consuming most of it and nothing being left for algae to grow on.

How to start the nitrogen cycle in a new aquarium

Generally, there are two methods on how to kick-start the nitrogen cycle in a new aquarium: the fish-less cycle (recommended for beginners) and the fish-in cycle.

Fish-less cycle

Here you start with just a few snails in your aquarium. They are more robust than other animals like fish or shrimp. You feed the snails 1-2 times per week. It's best to feed a little too much rather than too little because the excess feed will help start the nitrogen cycle by being a food source for the beneficial bacteria.

Check water parameters daily, especially ammonium and nitrate. Also do regular water changes. As soon as you notice a spike in either ammonium and nitrite, follow instructions in the troubleshooting section below.

Once you've had the aquarium running for 4-8 weeks and don't notice any spikes in ammonium or nitrite anymore, your tank is fully cycled. You're ready to add some fish to your aquarium safely. But don't overdo it because the bacteria present in your tank won't be able to deal with too much fish waste at first. Therefore, it's best to start with a lower number (e.g. 5 fish) and add new animals gradually.

Fish-in cycle

If you're more experienced you can go for the fish-in cycle. You proceed the same way as with the fish-less cycle, but add a small number of fish right from the start (e.g. 2-5 animals at first).

Check the water parameters daily and act immediately if you notice a spike in ammonium or nitrite. We recommend using a proper water testing kit for this scenario, as regular test strips are usually not sensitive enough to alert you of the smallest changes.

Pro-tip: If you're fish seem to be grasping for air at the water surface after a few days, there is a high chance that your nitrite level is starting to spike.

What to do during a nitrite or ammonia spike

In case you an aquarium with no livestock, you don't need to worry about ammonia or nitrite. Only if you see high levels of nitrate (which should be very unlikely) we recommend a big water change to prevent algae bloom.

If you do have livestock, you need to react immediately when noticing ammonia or nitrite, as these are toxic for livestock. We recommend to do a big water change > 50% first and then try to buy live beneficial bacteria or an ammonia / nitrite remover.

Keep in mind that live bacteria need to be cooled in the fridge, otherwise they quickly become useless.

Daily checks of water parameters are mandatory for a couple weeks after noticing an ammonia or nitrite spike.

What to keep in mind during aquarium maintenance

When removing the sludge from your aquarium bottom / substrate or filter you always also remove the beneficial bacteria. This cannot be prevented when cleaning the aquarium.

In order to have the smallest impact as possible on the nitrogen cycle, we recommend not to clean your aquarium and filter at the same time.

It's best to clean the aquarium bottom with a gravel cleaner and clean the filter a month or two later. In case you have a big aquarium, it may also make sense to split the aquarium cleaning. One half is cleaned in the first month and the second half a month or two later. By doing so you make sure that the impact of cleaning is as small as possible on the nitrogen cycle.

This kind of deep clean is not part of the weekly or monthly maintenance and the filter / aquarium bottom only need to be cleaned once or twice a year.

Frequently asked questions

How long does the first aquarium nitrogen cycle take?

The duration of the cycling process usually varies between 2 to 8 weeks.