Understanding Nitrogen Cycle in Aquarium for Beginners

nitrogen cycle in aquarium

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Whether you’re a beginner or experienced aquarist, it is important to under the process of the nitrogen cycle if you want to keep your aquarium clean and its inhabitants healthy. While nitrogen may not be a problem in natural water bodies, it can cause problems in a manmade environment. In fact, the nitrogen cycle in an aquarium is essential to prevent health problems for your fish.

Because of the small space and overcrowding, there is a higher risk of nitrogen pollution in tanks. If you don’t manage this well or understand the cycle, you may cause illness or even death to your little friends.

This is all you need to know about the nitrogen cycle and its timeline in a home aquarium.

What is the Nitrogen Cycle?

In the world of aquarium keeping, the cycle refers to the process involving natural food creation (specifically microorganisms and plants), fishes eating them and producing waste, then nature again breaking them so the cycle can repeat itself.

However, the most relevant stage you might want to know about is the part where fish waste becomes toxic nitrogen compounds. These include ammonia, nitrite, and nitrate, which can be dangerous in high levels.

Of course, such a system is completely natural and should not require our intervention. But the nitrogen cycle can pose a hazard in a tank environment. It is because the waste forms faster than it can be converted into nitrates to be absorbed by the plants.

Nitrogen Cycle in Aquarium Timeline

How to Cycle a Fish Tank
Graphic by reefstable.com

There are 3 main stages in the cycle, each of which contains a basic compound of ammonia, nitrite, or nitrate.

A new aquarium doesn’t have any nitrogen-converting bacteria yet, so it takes time to foster this biological filtration system.

Stage 1: Ammonia

In the first stage, the fish first enters the new aquarium. Their waste and uneaten foods will break down into ammonia (either ionized or unionized). The presence of ionized ammonia is possible when the water pH is less than 7, and these are less dangerous for your fish.

On the other hand, unionized ammonia can be dangerous when there is no proper bacteria colonies to consume these compounds. Without the bacteria, ammonia levels will spike and encourage dangerous bacteria to grow.

The ammonia content will continue to rise until the nitrogen-converting bacteria forms. Before then, your tank may turn cloudy and smoky. Once the bacteria have formed and ammonia levels start to decrease, it means you are entering the second stage.

Stage 2: Nitrite

As enough Nitrosomonas bacteria become present to oxidize ammonia, they will turn them into nitrite. While the toxic ammonia content will decrease, nitrite will increase. Unfortunately, these are also toxic for your fish!

More Nitrosomonas means more nitrite, which in turn prompts the growth of the nitrite-hungry Nitrobacter. The nitrite levels in the aquarium will start to stagnate or fall only once there are more of the latter bacteria. It is the point when nitrite is consumed faster than they are produced.

Additionally, nitrites also one of the biggest concerns for aquarists. This is the most important part of the nitrogen cycle you need to be wary of! So, how can you prevent too much nitrite from building up?

The easiest way is to only feed your fish occasionally and remove any uneaten food from the water. Make sure you’re not overcrowding the aquarium either. Even if you think of some species like catfish and algae eaters as “cleaners”, remember that they still produce urine and feces too.

We also recommend conduction partial water changes using well-aged water. This will prevent excess build up of toxic compounds. However, never change more than 20% of the total volume of water.

Stage 3: Nitrate

In the last stage of the nitrogen cycle, Nitrobacter can convert nitrite into nitrate. While this compound is the least dangerous, it doesn’t mean you can just sit back and ignore it. They can also be toxic for most fish once the levels reach 20ppm.

When nitrogen compounds are oxidized in an aquarium, the final product would be nitrates. Everything from urine and fish waste to excess food, as well as remains of dead animals and plant matters, will become nitrate in the end.

Like the previous stage, you can prevent build up by regular partial water changes every two weeks and not overcrowding the tanks. You can also remove substances like fish waste and uneaten food to avoid unnecessary nitrogen compounds.

Furthermore, it helps to test the nitrate levels every month or so.

How Long is the Nitrogen Cycle?

In general, you can cycle new aquariums between 2 and 6 weeks, while some slower processes can take up to 2 months.

It’s more important in a new tank to keep a close eye on the cycle. You can do this by using a test kit to check the levels of ammonia, nitrite, and nitrate.

Remember: ammonia levels rise first, then drop once nitrite appears. Meanwhile, nitrate will not form until there is enough nitrite.

The timeline for the nitrogen cycle in an aquarium varies based on several factors. For instance, how much ammonia is produced, the efficiency of this biological filter, the presence of live plants, and whether you’re using additives to boost the bacteria’s presence.

How to Cycle Your Aquarium?

How To Nitrogen Cycle for Freshwater Aquarium
Graphic by aquaticexperts.com

On to the most important question. How can you actually perform a nitrogen cycle in your aquarium? Let’s take a look at two different methods: with and without fish.

Cycling Your Tank with Fish

We don’t generally recommend cycling your tank with fish, as this means exposing them to high ammonia and nitrite levels. There’s a risk of stressing them out and causing unnecessary diseases. Of course, some fish species are more hardy, so it’s better to use them.

Your main goal here is to introduce fishes that can survive dangerous levels of ammonia and nitrite until the good bacteria develops. Add only 2 fishes at most for every ten gallons of water. Any more can cause a hazardous spike.

Some fishes you can use include minnows, Zebra Danios, X-ray Tetras, and Gouramis.

In the meantime, feed your fishes sparingly so they do not produce too much waste. Remove any uneaten food and do not forget to change the water partially every 3 days. Finally, prepare an aquarium test kit to keep track of the various compound levels. Once they rise and fall back to zero, you will know that the process is done.

The last step is to add more fish! This is safe to do if the toxin levels hit zero. You should only introduce one or two fishes at the same time or you might risk a sudden ammonia spike. Yes, it can be a slow process, but a very crucial one!

Cycling Your Tank without Fish

Most hobbyists prefer to cycle their aquarium without fish as it is safer and more humane. In fact, it’s actually quite simple and easy to do!

First, add some fish flakes into your fishless aquarium every 12 hours. As the uneaten food decomposes, it will release ammonia into the water. Monitor the levels until it reaches at least 3ppm and repeat.

You want to encourage Nitrosomonas bacteria to develop and multiply, which they can only do if there is enough ammonia in the water. Do this for an entire week.

Continue until you start testing for nitrites and eventually, nitrates. Don’t forget to keep adding fish food all the while. Your cycle is almost complete when nitrates begin to form.

After that, it is time to wait for the ammonia and nitrite to vanish entirely. Just make sure that your nitrate doesn’t read higher than 40.

The final step is to start introducing your fish. Do this slowly with just a few fishes at a time. Otherwise, you could end up ruining the balance and spiking the ammonia levels.

Can You Speed Up the Nitrogen Cycle?

You don’t have the patience to wait for 6 weeks, and that’s okay. You can try a few tricks to speed up the cycling process, although you may need an established tank for this.

The first option is to use gravel from an established aquarium. There are bacteria attached to these gravels that can help speed up the cycle in your new aquarium.

You can also get the filter you’re planning to use in the new tank and run it in an established tank. Leave it there for a week and the bacteria there will “colonize” your new filter, before installing the same filter in your new tank. It also helps to have live plants in your tank, as they can help speed up the process.

One thing to keep in mind is that there is always a risk of transferring harmful pathogens or bacteria from your established tank to the new one. Make sure you use only an aquarium that you know is healthy and safe from contaminants.


At the end of the day, the nitrogen cycle in a saltwater aquarium, or even a freshwater one, is part of the natural process. If you’re aware of the stages and what the risks are, you will be completely fine.

The cycle is not rocket science, but it is still important to understand the process, especially if you are a beginner hobbyist. With that said, don’t worry too much and just have fun. Good luck!

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