The Aquarium Nitrogen Cycle: How To Cycle Your Tank

Are you a proud fish owner, but struggling to keep your aquarium water clean and healthy? Well, have you heard about the nitrogen cycle in aquariums? It may sound like a complex scientific concept, but it’s actually a crucial process that keeps your fish happy and healthy. 

In this article, we’ll break down the nitrogen cycle in simple terms, explain why it’s important, and give you tips on how to maintain a healthy aquarium for your aquatic friends. So, let’s dive in and learn about the magic of the nitrogen cycle!

the nitrogen cycle infographic

What Is the Nitrogen Cycle in An Aquarium?

The nitrogen cycle is a natural process that occurs in all aquatic ecosystems, including aquariums. It involves the conversion of ammonia, a toxic substance excreted by fish and other aquatic creatures, into nitrate, a less harmful substance. This process is carried out by beneficial bacteria that inhabit the aquarium’s filter media, substrate, and other surfaces.

The nitrogen cycle in an aquarium consists of three stages: the first stage is the production of ammonia, which is excreted by fish and other aquatic creatures as waste. The second stage involves the conversion of ammonia into nitrite by nitrifying bacteria, and the third stage is the conversion of nitrite into nitrate by another group of beneficial bacteria.

The nitrogen cycle is a critical process in maintaining a healthy and thriving aquarium. Ammonia, the initial product of fish waste, is highly toxic to fish and other aquatic creatures. If left unchecked, it can quickly accumulate in the aquarium water and cause significant harm to the inhabitants. 

Nitrite, another byproduct of the nitrogen cycle, is also harmful to aquatic life, albeit less toxic than ammonia. Nitrate, the final product of the nitrogen cycle, is relatively harmless in small quantities and can be removed from the water by regular water changes.

In an established aquarium, the nitrogen cycle helps to create a self-sustaining ecosystem that requires minimal maintenance. Beneficial bacteria that carry out the nitrogen cycle thrive in the aquarium’s filter media, substrate, and other surfaces, ensuring that ammonia and nitrite are continuously converted into nitrate. Regular testing of the water parameters is necessary to ensure that the nitrogen cycle is functioning correctly.

What Are the 3 Stages of The Nitrogen Cycle for Aquarium?

The nitrogen cycle is an important and complex process that helps to maintain a healthy living environment in aquariums. This cycle consists of three stages, each stage carried out by different groups of beneficial bacteria. These stages are:


Ammonification is the first stage of the nitrogen cycle and takes place when organic matter, such as animal excrement or decaying plant material, breaks down into ammonia (NH3). Ammonia is highly toxic to fish, so it needs to be eliminated quickly.


In this stage, beneficial bacteria convert NH3 into nitrites (NO2) which are slightly less toxic than ammonia but still dangerous for your fish. The nitrifying bacteria use oxygen (O2) in order to turn NO2 into nitrates (NO3), which are relatively harmless. This step requires aeration in order to run properly.


Denitrification is carried out by anaerobic bacteria, which consume NO2 without requiring oxygen and converts it into N2 gas which escapes back into the atmosphere. In nature, denitrification typically occurs in areas where oxygen levels are low – like at the bottom of ponds or streams where there may not be enough oxygen present for nitrifying bacteria to thrive. Aquarists can simulate this process in their tanks by adding specialized filters that provide anoxic environments for denitrifying bacteria to flourish.

These three stages work together to ensure that your tank water stays within safe limits for your fish and other aquatic creatures! Understanding how they work is essential to achieving a healthy aquarium environment that you can enjoy for years to come!

The Aquarium Nitrogen Cycle_ Complete Guide & Infographic

How Do You Cycle a Fish Tank?

To ensure the health and well-being of the fish and other aquatic organisms in your tank, it is crucial to establish a stable nitrogen cycle. This process involves establishing a colony of beneficial bacteria that will break down waste and toxins your fish produces, converting them into less harmful compounds that can be removed through routine maintenance. 

Here’s how to cycle a fish tank:

Set Up The Tank

Before cycling your tank, make sure it is properly set up with all the necessary equipment, such as a filter, heater, and thermometer. Add dechlorinated water to the tank and any necessary water conditioners to adjust the pH and hardness levels as needed.

Add Ammonia

The first step in cycling a fish tank is to introduce a source of ammonia, which will feed the beneficial bacteria that will eventually colonize your tank. You can add pure ammonia or fish food to start the cycle.

Add A Biological Starter

Once you’ve added ammonia, you should also add some biological starter to the tank as well. This contains the beneficial bacteria, and it can speed up the process of the nitrogen cycle.

Test the Water

As the ammonia is being processed by the bacteria, the levels of ammonia and nitrite will begin to rise, while nitrate levels will remain low. Regularly test the water with a high-quality test kit to monitor the levels of these compounds and adjust as needed.

(Find out about the best water for your fish tank!)

Wait For The Cycle To Complete

It typically takes 4-6 weeks for the nitrogen cycle to complete and for the bacteria to fully establish in the tank. During this time, be patient and avoid adding any fish or other livestock to the tank until the cycle is complete.

(Have you ever wondered if it’s possible to cycle your fish tank in 24 hours?)

Perform Water Changes

As the cycle progresses, you may notice a buildup of nitrate in the water. This is a good sign that the cycle is working, but it is important to keep nitrate levels in check to avoid harming your fish. Regular water changes will help to reduce nitrate levels and keep the water clean and healthy for your fish.

Add Fish

Once the cycle is complete, and the water conditions are stable, it is safe to add fish to your tank. However, be sure to start with a small number of fish and gradually add more over time to avoid overloading the tank and causing a new cycle to begin.

how to cycle your aquarium

How Do I Know My Tank Is Cycled?

The process of cycling a fish tank is essential for establishing a healthy and safe environment for your aquatic inhabitants. But how do you know when the cycle is complete and your tank is ready? Here are some tips that can help

Test the Water Parameters

One of the most reliable ways to determine if your tank is cycled is by testing the water parameters regularly. Use a high-quality test kit to measure the levels of ammonia, nitrite, and nitrate in the water. In a cycled tank, the ammonia and nitrite levels should be close to zero, while the nitrate level should be no more than 20ppm. This indicates that the beneficial bacteria have established and are breaking down waste and toxins effectively.

Monitor the Cycling Process

During the cycling process, you may notice a spike in ammonia levels, followed by a spike in nitrite levels, and finally a reduction in both levels as nitrate begins to accumulate. This is a sign that the beneficial bacteria are colonizing and converting the toxic compounds into less harmful ones.

Smell the Water

Another way to tell if your tank is cycled is by smelling the water. A fully cycled tank should have a fresh and clean smell, while an uncycled tank may have a strong ammonia or fishy odor.

Observe the Behavior of Your Fish

When your tank is cycled, your fish should appear healthy and active, with no signs of stress or illness. If you notice any unusual behavior, such as lethargy or loss of appetite, it could be a sign that your tank is not cycled or that there is another issue affecting the water quality.

Check the Clarity of the Water

In a cycled tank, the water should be clear and free of debris or cloudiness. If you notice cloudy water or excessive algae growth, it could be a sign that the tank is not cycled properly or that there is an imbalance in the water chemistry.

Fish In Vs Fishless Cycle

How To Speed up The Cycling Time of Your Fish Tank?

Cycling a fish tank is a necessary and essential process for providing your aquatic inhabitants with a safe and healthy environment to live in. Unfortunately, this process can take some time before it’s complete – but what if you could speed up the cycle and get everything running smoothly faster? Here are a few tips on how to do just that:

Start With Dechlorinated Water

When filling your empty tank with water, make sure to use dechlorinated or pre-cycled water. This will help give your beneficial bacteria a head start over free-floating ammonia particles, so they’ll have an easier time establishing the nitrogen cycle in your aquarium right off the bat.

Seed Your Tank with Beneficial Bacteria

One of the most effective ways to speed up the cycling time of your fish tank is by seeding it with beneficial bacteria. You can do this by adding a small amount of filter media or substrate from an established tank to your new tank. This introduces the necessary bacteria to break down waste and toxins and can significantly reduce cycling time.

Add Ammonia to the Tank

Adding ammonia to your tank can also speed up the cycling time by providing a food source for the beneficial bacteria. You can do this by adding pure ammonia or by adding fish food or other organic matter to the tank. This simulates the waste produced by fish and jumpstarts the growth of beneficial bacteria.

Increase the Temperature of the Tank

Beneficial bacteria thrive in warmer water, so increasing the temperature of your tank can also speed up the cycling process. However, be sure not to exceed the recommended temperature range for your fish, as this can stress them and lead to health problems.

Use a Cycling Accelerator Product

Several products on the market claim to speed up the cycling time of your fish tank. These products contain beneficial bacteria and other additives that promote the growth of beneficial bacteria and can reduce cycling time. However, be sure to research and choose a high-quality product from a reputable manufacturer.

Common Cycling Problems and How to Fix Them

Water cycling is an essential part of keeping an aquarium healthy and balanced. It involves water movement through various aquarium components to ensure proper filtration, aeration, and circulation. However, water cycling problems can arise, and if not addressed, can lead to serious consequences for your aquarium inhabitants.

In this article, we’ll take a look at some of the most common water cycling problems and how to fix them.

Problem #1: Cloudy Water

Cloudy water is a common problem that many aquarium owners experience. It is usually caused by an excessive buildup of waste, uneaten food, and other debris in the tank. This buildup can cause bacterial blooms that result in cloudy water.

Solution: To fix cloudy water, first, perform a partial water change of around 10-20% of the aquarium water. Next, ensure that you are not overfeeding your fish, as uneaten food can cause debris buildup.

You should also check your filter at this time to ensure that it’s working properly.

Problem #2: Algae Growth

Algae growth is another common water cycling problem. It can occur due to various factors, including excess nutrients, poor water circulation, and prolonged exposure to light.

Solution: To fix algae growth, first, ensure that you are not overfeeding your fish, as excess nutrients can contribute to algae growth. Next, consider reducing the amount of light your aquarium receives, as too much light can stimulate algae growth. You can also add live plants to your aquarium, which can help absorb excess nutrients and compete with algae for resources. 

Finally, consider adding a protein skimmer or other filtration system to help remove excess nutrients from the water.

Problem #3: Nitrite or Ammonia Spikes

Nitrite and ammonia spikes can be deadly for aquarium inhabitants. They typically occur during the early stages of the aquarium’s cycling process, when beneficial bacteria have not yet established themselves to convert these harmful compounds into less harmful ones.

Solution: To fix nitrite or ammonia spikes, perform partial water changes of around 20-30% of the aquarium water. Next, ensure you are not overfeeding your fish, as excess food can contribute to ammonia spikes.

Ensure that your filter is working properly, and that the biological media is big enough for your tank.

During this time, you can also use an ammonia neutraliser like API Ammo Lock, to make any ammonia in your tank harmless to your fish.

Lastly, test your water regularly to monitor nitrite and ammonia levels and take appropriate action if levels become too high.

Problem #4: Low Oxygen Levels

Low oxygen levels can be a serious problem for aquarium inhabitants, especially in heavily stocked tanks or those with poor circulation.

Solution: To fix low oxygen levels, consider adding an air pump and air stone to increase water circulation and oxygenation. You can also add live plants to help oxygenate the water during the daytime. Finally, ensure that you are not overstocking your aquarium and that your filtration system is working correctly, as excess waste can contribute to low oxygen levels.

How To Deal with New Tank Syndrome

New tank syndrome is a common ailment among aquariums that can cause fish to become ill; this is due to the fact that the nitrogen cycle needed for a healthy environment has not been properly established. Here are some tips on dealing with new tank syndrome and getting your tank up and running in no time.

Test Your Water

The first step in dealing with New Tank Syndrome is to test your water regularly. Use a water testing kit to monitor ammonia, nitrite, nitrate, and pH levels. If you notice high levels of ammonia or nitrite, it’s a sign that the beneficial bacteria have not yet established themselves.

Perform Partial Water Changes

To reduce ammonia and nitrite levels, perform partial water changes of around 10-20% of the aquarium water. This will dilute the concentration of harmful compounds and provide a fresh supply of oxygen to the aquarium.

Limit Feeding

Overfeeding can contribute to New Tank Syndrome by producing excess waste that the beneficial bacteria cannot keep up with. Limit feeding to once or twice a day, and only feed what your fish can consume in a few minutes.

Add Beneficial Bacteria

One way to establish beneficial bacteria in the aquarium is to add it directly. Some products contain live bacteria cultures that can help speed up the nitrogen cycle process.

Be Patient

The most important thing to remember when dealing with New Tank Syndrome is to be patient. It takes time for the beneficial bacteria to establish themselves and rushing the process can do more harm than good. Regularly test your water and perform partial water changes as needed until the nitrogen cycle stabilizes.

Preventing New Tank Syndrome

The best way to deal with New Tank Syndrome is to prevent it from occurring in the first place. Here are some tips for preventing New Tank Syndrome:

Cycle Your Tank

Cycling your tank is the process of establishing the beneficial bacteria necessary for a healthy aquarium. This process can take anywhere from a few weeks to a few months, depending on the size of your tank and other factors. 

During the cycling process, you’ll need to add an ammonia source, such as fish food or pure ammonia, to the tank to provide the bacteria with the necessary food. Be sure to test your water regularly during the cycling process to monitor ammonia, nitrite, and nitrate levels.

Add Beneficial Bacteria

Adding beneficial bacteria supplements can help speed up the cycling process and establish a healthy bacterial population in your tank. These supplements are available in various forms, including liquid and powder, and are safe for fish and other aquarium inhabitants.

Don’t Overstock Your Tank

Overstocking your tank can lead to excess waste, which can overload the beneficial bacteria and lead to New Tank Syndrome. Stick to recommended stocking levels for your tank size and avoid overcrowding your tank.

Avoid Overfeeding

Overfeeding can produce excess waste that the beneficial bacteria cannot keep up with, leading to toxic conditions in your aquarium. Feed your fish once or twice a day and only feed what they can consume in a few minutes.

Maintain Your Filtration System

Your filtration system is vital for maintaining a healthy aquarium. Be sure to clean and maintain it regularly to ensure that it’s working correctly. Replace filter media as needed, and clean the filter housing and impeller regularly to prevent clogs.

Perform Regular Water Changes

Regular water changes are essential for maintaining a healthy aquarium. They help remove excess waste and pollutants from the water, dilute harmful compounds, and provide a fresh supply of oxygen to the aquarium. Aim to perform partial water changes of around 20-30% of the aquarium water every one to two weeks.


Here are some frequently asked questions people have about the nitrogen cycle in their aquarium.

Do You Have to Cycle a Fish Tank?

The answer is yes – cycling your fish tank is essential for the health of its inhabitants. Cycling is the process of establishing a beneficial bacterial population in the aquarium that can help convert ammonia waste into nitrate. Without cycling, levels of ammonia can quickly rise to dangerous levels and cause fish to become seriously ill or die.

Does Algae Mean Your Tank Is Cycled?

No – while algae growth in a new tank can indicate that some bacteria have started to establish themselves, it does not necessarily mean that the tank has reached full cycling capacity yet. In order to be sure that your tank is fully cycled, you’ll need to check the water parameters on a regular basis to ensure they’re stable and within healthy ranges.

Can You Cycle a Tank in 2 Weeks?

In most cases, it’s possible to cycle an aquarium in 2 weeks; however, it will vary depending on the size of the tank and type of filter system used. It’s important to note that some tanks may take longer than others so don’t be discouraged if yours isn’t fully cycled after two weeks – simply continue testing water parameters regularly until you see signs that suggest your tank has established its good bacteria colony adequately.


In conclusion, understanding and maintaining the nitrogen cycle in your aquarium is vital for the health and well-being of your fish. By providing a suitable environment for the beneficial bacteria to thrive, monitoring water parameters, and performing regular maintenance, you can ensure a happy and healthy life for your aquatic friends.

Don’t let the science jargon scare you away, just remember the basics of the nitrogen cycle, and you’ll be well on your way to a successful aquarium journey. Happy fishkeeping!

About the author

Hey! I'm Antonio!

Betta fish keeper for over 6 years now! Since owning a betta I've also housed all kinds of tropical fish, and have seen all manner of problems and how to look after them!

If you need any advice you can always message me or better yet join the Facebook group where a community can answer your questions!

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