Nitrogen Cycle In A Fish Tank – What It Is And How To Cycle A Fish Tank

Are you looking forward to starting your first fish aquarium? Well, you might have come across the word or heard something to relate to the ‘fish tank nitrogen cycle.’ It’s followed by a plethora of scientific graphs and terms, which seem a bit overwhelming in most scenarios.

That’s why I am here to help. This article will detail the nitrogen cycle in a fish tank and how to cycle a fish tank highlighting all the basic processes and compounds that comprise this cycle. So, without further ado, let’s get cracking.

What Is A Nitrogen Cycle?

The organic Nitrogen Cycle is a comprehensive cycle involving Nitrogen moving from the air to the plant, the animal, the bacteria, and back to the atmosphere- such a cycle requires no human interference.

However, in a fish aquarium, this Nitrogen process is a lesser a cycle and more a biochemical process, which incorporates the continuous chemical breakdown of nitrogenous compounds, starting with ammonia, then nitrite, before forming nitrates.

Since nitrates are produced last, they are mainly absorbed by the aquarium plants or eliminated from the aquarium’s water, using other means.

This cycle explains how organic wastes in the water are processed in the natural environment. Even if you possess a closed aquarium, this process should be initiated.

The prevalent biological toxins present in an aquarium are nitrate, nitrite, and ammonia. Therefore, the nitrogen cycle should operate efficiently to metamorphose and reduce this byproduct waste in the water.

What Role Does It Play in A Fish Tank?

Aquarium wastes decompose to produce ammonia, which is acutely toxic. The nitrogen cycle converts ammonia to nitrates through a multistage reaction. Nitrate, on the other hand, is much less harmful than ammonia and nitrite. So the nitrogen cycle is crucial.

In a functional aquarium, this process is well-established with time. Often, it takes about three months before the new aquarium can convert the waste byproducts into nitrates fully.

Undoubtedly, this toxic environment is extensively harmful to your fish, meaning that most fish will not survive without it—those who are quite vulnerable to illnesses won’t live long.

The process of replenishing a new aquarium gradually over time with smaller, younger fish or starter fish allows the Nitrogen converting bacteria to grow and regulate the load of waste products present in the water. Even though this Nitrogen Cycle can work without putting in any individual effort, it doesn’t guarantee that the new fish will survive inside.

Different Stages of Establishing Nitrogen Cycle in Fish Tank

There are several ways to cycle fish, but before I get to that, let’s find out about Ammonia, Nitrite, and Nitrate’s role across different stages in this cycle.


In normal circumstances, the ammonia is colorless and has a pungent smell that’s quite toxic. Bacteria instantly metamorphose fish proteins and urea into ammonia. But once the ammonia levels go way above, either there are too many fish, or fish present are being overfed.

Nevertheless, in a balanced aquarium, the bacteria, also known as ‘nitrogen-fixing bacteria,’ proceed to oxidize the ammonia and convert it into nitrite.


Nitrite causes the most fatalities to fish in an aquarium; hence these compounds should be effectively left out of the Nitrogen Cycle. Nitrites develop in the fish tank as a result of incomplete oxidation of the ammonium ions.

The nitrite-converting bacteria proceed to convert these nitrite compounds to Nitrate, thus making it generally harmless.


Nitrate is the final product in the oxidation of ammonia compounds, i.e., foodstuffs, urine, excretes, remains of snails, plant leaves, and other dead fish. In a fish tank, the nitrates produced mainly result from ammonium compounds and animal proteins’ disintegration.

Most tropical freshwater fish and other animals present in an aquarium are highly-tolerant to even large nitrate quantities. But as with nitrite, to avoid excessive build-up of nitrates, you should maintain a small animal population and feed lightly.

nitrogen cycle in fish tanks

How Long Does the Nitrogen Cycle Take?

It takes six to eight weeks, typically to achieve a fully cycled tank. But this period may vary depending on some factors. If you add ammonia additives (fishless cycling) or have plants for cycling, the required period may reduce to four weeks. So ideally, in a four to six weeks period, you can get a fully cycled aquarium.

How to Cycle A Fish Tank (Step by Step)

All new fish tanks must undergo the highly-involving nitrogen cycle. Although there is no means through which you can immediately establish a required and balanced nitrogen cycle in an aquarium, there are several ways to do so, but all will need time.

Typically, some are easy than others, but based on my research and extensive experience in running multiple aquariums, the following methods have served us well.

What is nitrogen cycle in a fish tank_

Method#1: How to Cycle A Fish Tank with Fish

This method is more familiar to both beginners and experienced fish keepers. Many fish keepers will not observe an empty aquarium for weeks or even months, hoping that some useful bacteria are thriving. To help you get started, this is what you should do:

Step-1: Begin with a limited fish population

When starting a new fish tank, begin with a limited fish population. The most recommended option is stoking a single fish per 10-gallon water tank. Check your stocking list, and select only the strongest and lasting species among them.

Step-2: Slowly raise the fish amount

Begin by feeding your fish sparingly and slowly raising this amount over the coming four to six weeks. The useful bacteria present will feed on the fish excretes.

But because there are not many bacteria present during the startup, you don’t wish to overfeed your animals to the point where there aren’t enough bacteria to handle the waste load fully.

Step-3: Add beneficial bacteria

You can significantly speed up the nitrogen cycling process by adding some useful bacteria at the beginning.

Preferably, if you already have an existing fish tank or know somebody who does, transfer some substrate or filter media from the established aquarium to this new one (more on this later in the article).

Besides, you can purchase live nitrifying bacteria to help kickstart your cycle.

Step-4: Check the water quality

Using a water test kit, you can check the water quality in your fish tank to ensure the animals are healthy and safe. Unfortunately, test strips are not accurate at all & you can’t be able to do all the tests you require. 

Therefore, I recommend API Aquarium Test Kit, which is a must need for any freshwater aquarium. It’s accurate & also much cheaper; you can perform almost 800 tests in one cheap pack. 

The cycle is deemed complete once you can comfortably feed your fish an equal amount for a week, and the nitrite and ammonia levels remain 0 mg/l, whereas nitrate levels stay above 0 mg/l.

Step-5: Gradually add more fish

You can gradually start adding some fish at this juncture, allowing some wait time to make sure this useful bacterium balances the increased waste load.

Note that even though Nitrate is much safer for the fish, if its concentration goes past 40 mg/l, it should signal you to perform a water change to reduce the nitrate levels.

Method#2: Cycle A Fish Tank Without Fish

This method of cycling has gathered popularity across the internet as it is beginner-friendly and relatively harmless. The advantage is that you don’t need fish, to begin with. But ensure you understand what you are doing so that you don’t struggle with completing the final process.

Step-1: Determine when your fish are set to arrive

Timing is highly crucial when performing fish-less cycling. If you are purchasing your fish online, not in local stores, contact your suppliers beforehand to determine when your fish are set to arrive.

Step-3: Introduce a suitable ammonia source

Place the fish food or any suitable ammonia source in an empty fish tank to allow bacteria growth. Continually add these ammonia sources to ensure your bacteria won’t die from lacking food. Invest in a water testing kit to monitor the levels of nitrites, ammonia, and water.

Step-3: Start adding fish

After your filter has been cycled, you must add some fish to maintain a functional system gradually. Like other bacteria-introducing methods, Fish-less cycling won’t cycle your aquarium immediately- it must take some time.

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If you’re comfortable with this cycling process, ensure you seed your aquarium with Nitrogen converting bacteria using bacteria additives or filter media to make it all easy for you.

Method#3: Cycle Your Fish Tank with Plant

This cycling method is loved by many because you can effectively convert your aquarium into a natural ecosystem, both visually and biologically.

Step-1: Add plants to the tank

Instead of setting up an aquarium with little or no fish, you instantly add some live aquarium plants (Check our article for the aquarium planting guide). The prime focus is on growing them using good fertilizers, substrate, and under good lighting.

Some microbiologists even claim that water plants effectively take in much more nitrogen waste compared to bacteria. Besides, the roots and leaves also contribute some useful bacteria.

Nonetheless, you should add even more using the tips highlighted in the previous section.

Step-2: Check the growth of plants

Once these plants start showing new growth, your cycle is done. Your plants are effectively taking in nitrates and ammonia and metamorphosing them into new roots and leaves.

Step-3: Test the water quality

Get a water test kit to ensure that nitrites and ammonia stay at 0 mg/l, whereas nitrates are not anywhere above 40 mg/l.

Step-4: Introduce fish

Slowly add some fish, ensuring you feed them lightly.

How to Cycle a Fish Tank Fast?

Establishing an efficient nitrogen cycling process in your fish tank might take you some time. Fortunately, there are some methods through which you can achieve fast cycling. These include using:

Method#1: Pre-Established Media

Also referred to as seeding your fish tank, investing in pre-established media from an existing system is the conventional way of kickstarting your beneficial bacteria in a new fish tank.

Use some fraction of filter media from another aquarium or gravel cup from a stable and place it into your filter.

The transfer between different systems should occur as fast as possible to prevent the bacteria from drying or dying. Except if you have much filter media to spare, this method shortens your cycling process and allows your filter to attain its full potential.

All in all, not all bacteria will effectively function across different aquariums. Avoid seeding material from a tank, which has significantly varying water parameters from your new tank.

Check your water’s kH and pH to ensure they fall in the same range. Like most fish, bacteria won’t support dramatic changes in water chemistry.

Every fish will introduce some form of bacteria into the tank. The bacterium you begin with may not be those you finalize with. Closely monitor your nitrogen parameters to ensure everything is running smoothly.

Also, never seed from an aquarium showing any signs of infections. Although most pathogens solely reside in the fish, chances are there may be some lurking in that filter.

Method#2: Dual Filters

Another way to speed up the nitrogen-cycling process is by placing the filter you are set to use in a new tank that’s already been established and allowing it to run shoulder-to-shoulder with the established filtration system.

This method requires some advanced personal planning. As others explained above, ensure your systems have kindred chemistry patterns and are healthy enough. In 4-6 weeks, you can transfer the matured filter to your new system.

Before going ahead, ensure you have everything set, and remember to add some little fish to the filter- it provides the needed food (ammonia).

Method#3: Bacterial Additive

Most fish stores offer bottles of bacteria colonies to jump-start your filter instantaneously. Regardless of whether what’s in that bottle is beneficial, the chances are that this bacterium might not be what your fish needs.

Save yourself the cash and spend some time setting up a fully-effective filter. Steer clear of the lure of immediate gratification and save your fish in the long run.

When Do I Know My Tank Is Cycled?

Keep testing your aquarium water. The indication of the tank being cycled is pretty straightforward. The ammonia and nitrites levels will start to fall, and the nitrate level will start rising in the cycling process. You’ll see the following signs in a fully cycled tank.  

  • The ammonia level will be zero.
  • The nitrite level will be down to zero.
  • The nitrate level increased and stayed steady. (The ideal Nitrate level in a fish tank is below 40 mg/l. If nitrate level rises further, perform water changes.)
  • Fish become more energetic and consume more food than before. (Fish-in Cycling)
  • Plants start flourishing. (Cycling with Plants) 

Test Kits

Testing the aquarium water is crucial. Especially during cycling periods, you have to frequently check pH, ammonia, nitrite, and nitrate levels. A reliable testing kit is vital to get an accurate result. I can suggest a testing kit box, which I found useful. With the API Aquarium Test Kit box, you can measure all necessary parameters during cycling.

Common Nitrogen Cycle Problems & Solutions

Cycling the tank is not a tough task. Mostly, it requires time and patience. Still, you might have faced some problems associated with cycling. Let’s explore those to be prepared to overcome if any of those arise.

Fish-in Cycling: Ammonia poisoning

Ammonia poisoning is a common problem when you are cycling a new tank with starter fish. Some people think starter fish are to be sacrificed, but I can’t entirely agree with this option. If you are careful enough, it is possible to complete a fish-in cycle without losing any fish.

There are two common possible reasons for ammonia surges:
!) High fish density.
!!) Overfeeding

In the early stages of the cycling period, ammonia levels can rise to affect the fish. An ammonia-poisoned fish will show some common symptoms like; being lethargic in movement, grasping for the air at the surface or sitting idly at the bottom, and losing appetite.

You may identify inflammation in the eyes and anus in severe cases, with red slashes in the fins. If you notice ammonia poisoning symptoms, take immediate action to change at least 40-50% of the water.

Related read: How to lower ammonia in a few simple steps.

Fishless Cycling: Ammonia is not converting to nitrates

There are three possible reasons for not achieving the desired ammonia drop in fishless cycling:
!) You are changing the water too frequently.
!!) The water you are using in the aquarium is not chlorine-free.
!!!) The aquarium water is of low pH value. (Acidic water)

If you are changing water too frequently, beneficial bacteria will not get the opportunity to grow their colony. Without useful bacteria, it is not possible to achieve the desired result.
Water chemistry plays a vital role when cycling a fish tank.

It is strongly advisable to ensure the water you are using for your aquarium is chlorine-free. Chlorine kills beneficial bacteria that convert ammonia to Nitrate.

If the water you are using in the aquarium is acidic, it will convert ammonia to ammonium compounds instead of converting to nitrite and nitrates. It is advisable to use a pH testing kit and ensure the water pH is above 6.5.

Cycling With Plants: Delayed cycling

The main reason for delayed cycling, when you have live plants in the aquarium, might be inadequate ammonia dosing. Cycling should start from the third day you started. But if it doesn’t start even on the fifth day, it is an indication that the plants are consuming all the ammonia you are dosing.

In such a scenario, you have to increase the ammonia dosing rate and perform water testing every other day. If the process doesn’t start, add more ammonia.

Common for All Methods: Algae bloom

The possible reasons for algae bloom during the cycling period are:
!) Overlighting
!!) Over-fertilizing
!!!) CO2 injection

If you observe excessive algae growth, lower the lighting. For planted tank cycling, it is not possible to keep it in the darkness. In that case, turn off the lights at night. Stop adding fertilizer and CO2, and that will help to control algae growth.

Tips For A Healthy Aquarium – Keep Running Nitrogen Cycle

Once a nitrogen cycle is established in the fish tank, it becomes a safe habitat for fish. But the nitrogen cycle is a continuous process, and it never ends in a healthy aquarium. In any way, if this nitrogen cycle is hampered, your aquarium’s health will be unstable. So it is crucial to maintain a steady nitrogen cycle in the aquarium. You can do it by:

  1. Do partial water changes routinely. 10-15% of water weekly and replace one-third of water once a month.  
  2. Avoid thorough cleaning of the fish tank. A thorough cleaning can destroy beneficial bacteria colonies. Without beneficial bacteria, the nitrogen cycle in the aquarium may collapse. 
  3. Perform regular water testing for pH, ammonia, and nitrate.  
  4. Keep an ideal fish density constantly, don’t overstock, and don’t make any sudden fish number changes.

Final Words

To sum up, this article has comprehensively discussed all you need to know on how to cycle a fish tank. By cycling your aquarium, you are giving your fish the best environment for not only survival but also living happily and healthily.

Always bear in mind that once kickstarted, the nitrogen cycle runs consistently throughout your fish tank. Regardless of your preferred cycling method, it would be best if you understood; that no trick method can instantly establish fully-functional bacterial colonies.

You need to have a good plan and invest time in it. Check your water chemistry often until you have a stabilized and fully cycled system. I recommend that you incorporate a testing kit as part of your wholesome maintenance routine.

Sujit Modak

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