In nature, the most abundant elements that make up biological matter namely carbon (C), nitrogen (N), and oxygen (O) get recycled by various chemical and biological cycles.

All of these cyclings take place among the lithosphere, atmosphere, and hydrosphere where they change from one form to another, but the principal elements (N, C, and O) always remain constant.

However, these cycles can also take place in an artificial environment if the required conditions are present which comprises mostly living organisms.

Like the major cycles, a version of the nitrogen cycle is carried out in an aquarium that revolves around the water, fishes, plants, and microorganisms.

By the end of this discussion, you will have a general understanding of how everything works, and how the nitrogen balance can be attained in your fish tank.

Why is the nitrogen cycle necessary for your tank?

In an artificial setting like an aquarium, the harmful waste product mostly found is ammonia (NH3), which the fish produce.

However, leftover food particles, especially protein-rich food, dead parts of plants, and the quality of the water also determine the levels of ammonia in the water.

But the main issue arises when the levels get higher. This is a very toxic compound that is disastrous even in small amounts. Ammonia should not be allowed to remain in the water in the slightest because it causes a lot of health issues for the fish.

At higher concentrations, the fish will die very quickly. Even the hardiest of fishes are not able to tolerate it for long.

Usually, most of the nitrogen content is removed from the water by changing or replenishing it. But there is another natural way to control ammonia levels, and that is by using microorganisms and plants.

However, it requires a careful understanding of how it is broken down or converted to different compounds that are less detrimental to life forms.

Harmful effects of ammonia

High levels of ammonia cannot be visually detected as its dissolved form is as transparent as water. This can cause a lot of problems unless you track its increasing concentration.

There are many test strips available on the market to help you detect with good accuracy. But what happens if the ppm of ammonia gets high?

Not only does it cause severe discomfort, but also physical changes, which will make the fish very sick, and most probably die.

Here are some signs and symptoms that you should be wary about-

  • Blood red gills
  • Darkening of the body color
  • Labored breathing (gasping for air)
  • Increased salivation
  • Dropsy
  • Lethargic movements
  • Swollen eyes
  • Inflamed anal section

How to cycle a fish tank?

In order to establish a proper working nitrogen cycle in your tank, you should know about the different chemical and biological processes that go on inside the water.

  • Firstly, ammonia is introduced into the water when fishes excrete it as it contains ammonia naturally as compared to urea and uric acid in mammals and birds.
  • The nature of ammonia depends upon the acidity of the water. If it’s below the neutral pH, then it is present as its cation- ammonium (NH4+). And if it’s neutral or above, i.e., basic, then it stays as ammonia (NH3).
  • This ammonia is broken down into another form of a nitrogenous compound called nitrites (NO2) by the process of partial oxidation by denitrifying bacteria- Nitrosomonas. Nitrates are as dangerous as ammonia.
  • The nitrites are then further broken down into nitrates (NO3) by another type of bacteria in the group of Nitrobacter (e.g., Nitrospirainopinata).
  •  These nitrates are not as harmful as other byproducts of ammonia, but higher concentrations (at 20 ppm and above) might still create problems. At this moment, manually changing the water might be the best option.
  • In the process of conversion, hydrogen ions are produced as a byproduct, which may decrease the pH of the water. This causes ammonia to become ammonium, which the bacteria cannot use.
  • Anyhow, some of the ammonia and nitrates are used up by plants, which act as fertilizers for their growth. The fish has much more tolerance to nitrates than nitrites, but it still can create problems at higher concentrations.
  • Finally, the remaining nitrates or nitrogen is removed from the water by doing renewals or changes.

The chart given below shows how each component is transformed and cycled in a fish tank.

How to cycle a fish tank
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This aquarium cycle graph might help you understand how the concentration of each compound affects the total nitrogen content of the water.

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As we can see from the graph above, the total nitrogen concentration level increases gradually at a constant rate, as time goes on. It only dips during a water change.

This constant is calculated from the average increasing concentration of all three compounds- ammonia, nitrite, and nitrate.

Anyhow, this graph denotes an aquarium cycle timeline where everything is ideal. This does not always happen in real life. The concentration of all these will vary with the tank and its state.

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But if we carefully analyze the graph, we can conclude that the levels of ammonia are lower than the rest, while the amount of nitrite is lower than the former and higher than nitrate.

And this happens once the compound starts to get converted into its oxidized forms. However, a little amount always remains in the water no matter how much it is broken down. So, it is always better to change the water at regular intervals.

This video demonstrates how this whole cycling works-

Marine or brackish water systems

Interestingly, marine biomes have a different kind of nitrogen cycle than freshwater tanks. Natural sea rocks and sand inhabit beneficial denitrifying aerobic halotolerant bacterias that can convert the nitrates directly into their gaseous form, which is nitrogen (N2).

This is much better than freshwater tanks because the nitrogen is automatically eliminated from the system without any hassle of manually removing it, which keeps the environment less toxic.

Cycling the fish tank

Now that we have understood the basic science behind it, let us see how to force a nitrogen cycle in our tank.

There are generally two basic ways of cycling nitrogen in a tank-

  • Fishless cycling
  • Fish-in cycling

Fishless cycling

In this type of nitrogen cycling, the main components are water, microbes, and plants. This is a very beginner-friendly approach, and one of the easiest ways to eliminate ammonia.

Introduction of ammonia

As there are no fishes included in this method, we first need to find a way to introduce ammonia into the system. And the best possible way is to use fish food.

Adding a few commercial fish foods like flakes or pellets, and letting them decompose is a great way of increasing ammonia levels in a short amount of time.

It takes a few days to amp up the concentration. However, be careful not to add too much, or it will take a long time to cycle it all.

Author Note- Another way of doing this is by adding weak acids such as ammonium chloride (NH4Cl). Even though it is a quick process, it is not recommended because it also produces aqueous chlorine alongside ammonia.

Check the ammonia levels

Now that we have established significant ammonia levels in the water, it is time to find out the exact amount. You can use specialized kits or strips to check if it is around 3 to 4 ppm concentration.

These kits are inexpensive and can easily be ordered from Amazon. API and Tetra are among the popular choices for an aquarium.

If it is below that threshold, you need to let the food particles decompose for a few more days to bring it up to the desired level. You must not let it go under 3 ppm.

You should keep this up for a few days to a week, making sure the ammonia levels remain stable.If not, then the bacteria will not grow properly.

If everything goes as planned, the denitrifying bacteria will start to colonize the water, and begin the process of breaking down the ammonia.

Detection of nitrites

Let us presume that everything went as planned, but we can only know if this method worked by checking the nitrite levels of the tank.

Similarly, nitrite test strips can easily be bought online or from a local store. The levels should indicate somewhere between 1 and 5 ppm.

If the detection test is positive, you will know that the ammonia has started to break down, and its levels are getting low.

Formation of nitrates

After a week or so, you will start to notice the nitrite and ammonia concentration dipping. This means that nitrite is being utilized by Nitrobacter, and getting converted into nitrates.

Essentially, you don’t need to check the nitrate levels because the decline in ammonia and nitrite is a clear indicator that nitrate is forming. But if you want to be sure, you can test it using similar strips.

So, when this happens, we can safely assume that our process has come to an end and that we have established all the necessary factors required to cycle ammonia naturally.

But, before we go on adding the fish, it is safe to keep doing this for another week or two, just to make sure that everything is in order, and in equilibrium.

Final step- Adding the fish

After everything strikes a state of balance, you can now safely add fish to the tank.

But initially, you should add only a few fishes, and start with the hardy ones first. You must give it some time, and add the remaining at a later date.

Pro tip- Remember to clean the excess decaying food matter before adding the fish. Or it might lead to an unexpected spike in ammonia and cause problems for the fish.

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Fish-in Cycling

This method of cycling involves fish during the whole process of bacterial colonization. This method is generally not advised because it can sometimes lead to ammonia poisoning and cause diseases. Many times, it might even lead to death.

It is also advised to de-chlorinate the water before adding the fish. Chlorine is harmful to fish and especially to bacteria.

Choosing the right fish for the job

So, in order to perform this you must carefully choose a strong, hardy fish that can tolerate slightly higher concentrations of ammonia than others. A few examples of such fish are-

  • Pupfish
  • Minnows
  • Guppies
  • Tiger Barb
  • Zebra Danio
  • Tetras (X-ray)

Pro tip– Add a maximum of 2 to 3 hardy fish during the initial stages. Having too many will cause the ammonia level to increase substantially, and the conversion to nitrite won’t be fast enough.

Feeding and water changes

Feeding intervals should be as minimal as possible. And the feed should be adequate so your pets do not starve.

The reason behind this is that excess food will not be eaten by the fish and it will start to decompose after a while. This, in combination with the waste produced, will cause ammonia to skyrocket.

Generally, during this time, fish should be fed once or twice every two to three days. This will ensure the water doesn’t become too noxious before the cycling takes place.

On top of that, water changes should be done every few days. About 25 to 30% of the water should be replenished in order to suppress extra toxicity.

Pro tip- Do not change the water regularly as it will cause the ammonia levels to decrease, halting the growth of bacteria.

Checking the status of ammonia and nitrite production

As described in the previous section of fishless cycling, you can check the ammonia concentration using a kit or strips.

Once it is around 3 to 4 ppm, the beneficial bacteria will start growing and begin the oxidation of ammonia. This will cause a dip in ammonia and a rise in nitrite.

Check the levels again to ensure everything is going smoothly. The nitrite-converting bacteria will start growing and producing nitrate from it.

Maintain this state for a week or two, until both the ammonia and nitrite levels start to decrease, and stay at zero or low levels constantly.

This means the bacteria have managed to maintain a stable situation with the environment, and toxins have started cycling properly.

Put in the remaining pets

Now that the lethality of nitrogenous compounds has been lowered, it is safe to add the remaining fish to the water and continue as usual.

However, you should always be aware of the waste produced afterward, and remove excess if needed. Vacuuming is the best way to do it.

Do not forget to change a portion of the water every few days to keep the toxic compounds as low as possible.

Tips and tricks to hasten the process

Even though the above-mentioned methods work quite well, especially the fishless method, there are also other ways to introduce natural cycling in the tank which may be faster and more efficient.

Substrate from the previous tank

If you already have a tank with a fully functional nitrogen cycle, then this procedure will help establish the cycle in the new tank much faster.

The substrate already harbors the essential nitrogen-converting bacteria, so you do not need to wait days or weeks for it to grow naturally without an inoculation.

Just transfer a part of that old substrate and mix it with the new one evenly, and just wait for a few days to give the bacteria time to adapt and flourish.

You will be able to get your tank recycling nitrogenous compounds in no time.

Filter from an old tank

As you know, the filter sieves out the particulate matter that makes the water murky and keeps it clean. All the gunk and stuff gets stored in the medium of the filter i.e., the sponge-like material inside the apparatus.

Beneficial Nitrosomonas and Nitrobacter can live in this medium. So, if you already have an old cycled tank, you can just transfer the filter whole to the current one.

After a day or two, the microorganisms will start to spread and multiply, and start the detoxifying process.

After a few weeks, you can choose to install a new filter and put the old one back. But only do it once you are confident that the water has enough microbial growth to support the cycling by itself.


Instead of transferring the old filter to the new tank, you can also do the opposite. Put the new filter into the old tank and let it run for a few weeks.

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This will help accumulate the essential cycling microbes, and then you can transfer the used filter back to the current one.

This process is much better because you won’t have to deal with installing another new filter to replace the old one.

Author note- You should be extra careful when you are dealing with an old tank filter. Alongside the beneficial bacteria, it might also contain pathogens. Do not try this method if you are unsure if the water is free of pathogens.

Choosing old vegetation

As we discussed above, plants play an important role in nitrogen cycling by utilizing nitrate as a nutrient substitute. This helps eliminate excess nitrate from the water.

And just like the filter, you can also opt to transfer plants from an already-established cycled tank.

The surface of the plants can inhabit those nitrogen-converting bacteria. And by introducing them to the new tank, you can speed up the process a lot.

However, the number of microorganisms will not be as high as the filter medium, but it is faster than waiting for spontaneous growth.

Alternatives to eliminate excess ammonia

You can buy water conditioners such as KordonAmquel, which can neutralize ammonia, nitrites, and nitrates present in the water. It can be very beneficial if there is a sudden increase in toxicity levels.

However, using a conditioner should always be used as a last resort. These chemicals do not remove them completely from the water. It just changes their chemical nature through a reaction. So, water changes are still the only viable option to remove excess ammonia.

Sings of a cycled aquarium

Let’s assume that you have successfully managed to set all your plans in motion. But how will you know that the cycle is successful?

  • The ammonia and nitrite levels have been consistently low (almost zero) for several weeks without the need to add any conditioner.
  • There are no signs of discomfort or discomfort in your fish or other aquatic pets as discussed above.
  • Low ammonia levels even after a two-week gap between water renewals
  • Higher nitrate levels as compared to very low nitrite and ammonia levels after several days of tests

Frequently asked questions (FAQs)

Why is there algal growth during freshwater tank cycling?

The cycling period is perfect for the growth of algae as the decomposing matter is rich in nitrogen and other nutrients. This can be a problem if not eliminated early.

You can pair algae eaters such as snails, shrimps, and some catfish species with the existing fish to remove algae during fish-in cycling.

In fishless cycling, the best way to eliminate algae is to deprive them of light. Just turn off the light till you see a decline in the green color of the water. Do this till it is clear.

How long does it take to cycle a tank?

There is no fixed answer to this. Some can take two to three weeks, while others can take more. It all depends upon the conditions and size of the tank. Bigger tanks will take more time than smaller ones.

But it also depends upon the environment and temperature. For the spontaneous growth of bacteria, the amount of ammonia and the temperature must be optimal.

In colder locations, this will take more time than usual than in warmer areas.

On top of that, if you can manage to introduce the microbes from your previous tank, this cycling takes a lot less time to complete.

Why are the ammonia levels not dropping?

There can be a few reasons behind this-

  • The amount of decomposing matter is too high for the bacteria
  • The acidity of the water is low
  • Chlorinated water

If, for some reason, the amount of decomposing matter elevates, it becomes harder to compensate if the bacterial load is low.

The acidity of water is responsible for high ammonia levels. If it is low, the ammonia is converted to its ionic state- ammonium, which cannot be taken up and utilized by Nitrosomonas. So it gets accumulated more and more.

Lastly, chlorine in the water will kill off the beneficial bacteria. So refrain from using tap water, or use a conditioner to dechlorinate the tank.


We hope that by now you understand how important the nitrogen cycle is for a tank and the various aspects you need to know to safely induce a natural cycle.

Without proper cycling, it will be difficult for your fish to survive, and you will have to do water changes at higher frequencies to keep them alive.

However, it is to be noted that larger tanks are easier to cycle than smaller tanks. Anything below a 10-gallon capacity will have problems.

So, make sure you have a 15-gallon tank at least, or you will have to do extra work to keep ammonia levels under control.

About the Author

Shelby Crosby

Shelby is a passionate fishkeeper who has been writing about fish for over 5 years. She is a pro aquarist and holds a BSc Honors Degree in Wildlife and Fisheries. She creates her own beautiful aquarium layouts and loves to share her knowledge of tropical fish with other hobbyists.

Career Highlights:

  • Has worked with several aquarium manufacturers as a consultant
  • Organized and hosted workshops on freshwater fish keeping at retail stores, educational facilities, and libraries
  • Released content for the amphibian community through her writings

Educational Highlights:

  • BSc Honors in Wildlife and Fisheries in 2011 (University of Northern British Columbia)
  • Completed her undergraduate thesis on the effects of zoochlorella supplementation on the growth and health of fish.

Writing Experience

Miss Crosby is a Freelance blogger; many of her articles are posted online on various blogs. She has also written a few short articles for "Tropical Fish Hobbyist Magazine" in the past. She is a regular contributor to Her education, first-hand experience with fishkeeping, and in-depth knowledge in aquaculture make her one of the most competent writers in the industry.

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