The Nitrogen Cycle (How to Cycle Your Aquarium)

Below you’ll find a step by step process on cycling your new aquarium.

We’ll break it down into a super easy format that even a beginner will be able to succeed.

If you’re sick of reading the old complicated beginner guides then check out our guide: Best Freshwater Aquarium Setup. Check it out


Tools To Help Cycle Your Tank

What Is The Nitrogen Cycle?

The nitrogen cycle is the biochemical process that turns inert or toxic forms of nitrogen into non-toxic and consumable components.

There are several processes in a nitrogen cycle, including nitrification, denitrification, nitrogen fixation, and decay.

Nitrogen is present in abundance in the atmosphere. Different kinds of bacteria in the surrounding environment convert it into consumable forms, mainly nitrites, and nitrates.

The nitrogen cycle takes place in the atmosphere, plants, the soil, and even in water. It is one of the most crucial processes in a fish tank.

Fish release ammonia (NH3) gas through their gills and their excretion. No living organism can survive when surrounded by the waste of their body excretes.

For fish, in particular, it is incredibly toxic and can lead to breathing problems and even death!

The nitrogen cycle in the fish tank is a natural filtration process that removes toxic ammonia from the surroundings by turning it into nitrates. In the process, toxic ammonia is converted into nitrite (which is also poisonous) and then to into the less harmful nitrate.

The Role of Beneficial Bacteria

The presence of beneficial bacteria is what fuels that nitrogen cycle in the fish tank.

These bacteria colonies regulate the nitrogen concentration, which is why you must ensure your aquarium has good bacteria in it before you include your fish.

Bacteria are not naturally present in the water, and you need to give it time to develop the system via the fishless cycling process or cycling with fish.

Lastly, the beneficial bacteria that will eventually develop in your filter will also help neutralize the toxic ammonia excreted by the fish.

The Role of Ammonia (NH3) And Nitrite

The level of Ammonia and Nitrite in the fish aquarium plays an important role.

Initially, you will introduce ammonia into the water. Once NH3 levels in the water rise to a certain extent, the bacteria that eat ammonia will start to grow.

The bacteria then convert ammonia into nitrite, which is also harmful to the fish. As the nitrite levels rise, the bacteria that eat it eventually start to turn it into nitrates that are less toxic for fish.

How Long Does The Cycling Process Take?

Use A Test Kit To Check Water Parameters

There are many retailers out there who claim the tank is ready to use in just a few days after you set up. That is not true and can be hazardous for your fish.

On average, it takes about four to five weeks for the process to complete. Before that, the aquarium is not inhabitable for fish.

The cycling process also gives you ample time to check whether all the equipment is working correctly, so you don’t have to stress about broken filters or air pumps.

The Nitrogen Cycle Time Line

There are three main stages of the nitrification process. First is the introduction of ammonia, second the birth of nitrite, and third is the conversion to nitrate.

Stage 1: Ammonia Levels

In the first stage, you will incorporate a source of NH3 in the water. You can add in fish waste, which releases NH3 almost immediately, or you can add in fish food, which also releases ammonia as it starts to decay.

You can also add in diluted household ammonia. The source will depend on the pH levels of the water.

Ammonia levels will continue to rise in the aquarium until enough good bacteria build up. The bacteria Nitrosomonas will eat the ammonia and produce nitrite.

Once NH3 levels start to decrease, you need to start monitoring the nitrite levels in the tank.

Stage 2: Nitrite Levels

Beneficial bacteria eat ammonia and produce nitrite as a byproduct. Because nitrite is also toxic to fish, you will have to monitor the nitrite levels in the tank carefully.

Once they reach a certain level, the bacteria, Nitrobacter, that eat nitrite, begin to increase and turn nitrite into Nitrates.

Stage 3: Nitrate Levels

Nitrate production is the last stage of the process.

Once the nitrate conversion process begins, you will have to monitor the nitrate levels in the aquarium. Though it isn’t as toxic in low concentrations, if nitrate levels cross 20-40 ppm, they start to become harmful for the fish.

You’ll know the process is complete when you have nitrate levels rising, and the ammonia and nitrite concentration reaches 0ppm (parts per million).

You will have to regulate NH3, nitrite, and nitrate levels through regular partial water changes.

Make sure to change around 20% to 50% of the water in the fish tank at least once every 1-4 weeks. You will also have to remove solid fish waste from the water regularly to control NH3 levels in the tank.

Related: Check out our article about the best nitrate removers you can use in your aquarium.

The 3 Ways To Start The Cycling Process?

There are multiple ways to start the process.

The two most popular ones are cycling with fish and cycling without. The third method is to use plants, which is a natural way and preferred by many.

Cycling With Fish

Use Hardy Fish When Cycling With Fish

The cycling with fish process involves five simple steps. Here the fish becomes the source of NH3, making the process a little tricky.

Step#1: Add A Few New Fish (Use Hardy Fish)

You will look for fish that can survive in high levels of ammonia, known as hardy fish.

Make sure to only add a couple of fish for every 10 gallons of water; if you add too many fish, they may end up producing too much NH3 through their waste.

Step#2: Feed Your Fish, So They Produce Waste

Feed your fish!

The more they eat, the more waste they produce, and the higher the ammonia in the tank. This is key in starting the cycle.

Note: Even hardy fish cannot survive in a lot of NH3 for an extended period. Make sure you feed the fish smaller portions once every two days.

Step#3: Water Changes

When cycling with fish, you will have to regularly change water to regulate ammonia and nitrite levels in the aquarium.

Change around 10% to 25% of the water once every two days to ensure just enough NH3 remains to fuel the bacteria production without being harmful to the fish.

Step#4: Test For Toxic Levels

Make sure you buy a testing kit to monitor the water parameters in the tank.

You’ll want to note the following;

  • Ammonia spike, then fall to 0ppm
  • Nitrite spike, then fall to 0ppm
  • Nitrates rise to 20-40ppm

You’re done! Nice work.

Step#5: Once Your Tank Has Cycled Add More Fish

Once the process is complete, you can add more fish to the aquarium.

Make sure you don’t overcrowd the tank or overfeed the fish. Too much waste production and decaying food can all create a toxic environment.

How To Complete A Fishless Cycle (May Need Weeks)

A fishless cycle takes longer than the one with fish because here, the NH3 levels remain the same as what you introduce initially; thus, the bacteria take longer to produce.

Step#1: Add Ammonia To The Tank

The simplest way to do this is to add fish food to the aquarium. Add in the amount you would if you were feeding the fish and top it up every 12 hours to speed up the process.

Once the flakes start to decay, they will release the gas, which starts the nitrification process.

Step#2: Use a Test Kit: Look For Ammonia, Nitrites & Nitrates

Monitor, the ammonia, nitrite, and nitrate levels to see where the cycle is.

It will also help you determine whether the fish food has started decaying or not. If the ammonia levels don’t rise, then you may need to consider another source.

Step#3: Ensure Your Tank Has Cycled (Important)

Before you add your fish, make sure the cycle is complete. Nitrite and Ammonia levels in the aquarium water should be at 0ppm.

Do not add in your fish before that because if you do, then within a day, gas levels in the aquarium can spike up to dangerous levels.

Step#4 Begin to Add Your New Fish

Add your fish in batches.

Again populating an aquarium to its full capacity can lead to spikes in gas and become hazardous for the fish. Wait at least a week or more before you add in the next batch and continue to monitor the conditions of the aquarium.

As long as you don’t add too many fish at once, you should be good to go.

Cycling Your Tank With Plants (My Recommended Method)

The best way to nudge the process along is to add live aquatic plants. If you can get some from an already established aquarium, then it is even better.

Live plants not only have good bacteria that can start the nitrification process, but they also moderate gas levels in the fish tank via protein synthesis.

Simply follow either method above (fishless or fish in) and just add plants. Plants have a lot of added benefits, like increasing time between water changes; they help remove harmful toxins naturally, which in turn benefits the fish during this process.

How To Quickly Cycle A New Aquarium?

Not many have the patience to wait 6-8 weeks for the cycle to complete. Here are a few techniques they can use to speed up the process.

Add Used Filter Media from Another Tank

Using filter media from an established aquarium is the simplest way to start the nitrification process. It will already have nitrifying bacteria in it and can introduce it in the aquarium to start the cycle quicker.

Adding Substrate From An Established Tank (Free)

You can also use gravel from existing fish tanks. Take about a cup of gravel and hang it in a mesh bag in your filter. The healthy nitrifying bacteria attached to the gravel will make its way into the tank.

Or, just lay the gravel from an established tank on the bottom of your new tank, and it can speed up the process.

Pre-Populate Your Filter With Beneficial Bacteria

You can also place new filter media in an established filter to allow the bacteria to seed your media. Then when you set it up in your aquarium, it will release bacterial colonies in the new tank without having to go through the ammonia introduction and degeneration phase.

Using Live Plants

Using live plants is the simplest and most hygienic method possible.

You can get them from an established aquarium to introduce bacteria in your aquarium even faster, or you can buy new real live plants to avoid the threat of cross-contamination.

Common Cycling Process Problems

There’s always a chance something might go wrong, and you may end up into some problems. It is still better to be prepared in advance than to let these problems catch you off-guard.

High Ammonia Levels

Ammonia is toxic, as we have established at several points in this article. It is highly likely that the levels of NH3 may rise to potentially dangerous levels, especially after you introduce your fish.

Watch out for:

  • Lack of movement
  • Low appetite
  • Inflammation in gills, anus, or around the eyes
  • Overcrowding at the bottom of the aquarium
  • Difficulty breathing – gasping for air at the surface.
  • Red streaks in the fins

If you notice any of these symptoms, start monitoring the conditions in the fish tank with a test kit, and conduct more frequent water changes.

Ammonia Is Not Dropping

NH3 should start dropping within 3 to 5 days. If that does not happen then a potential reason can be:

  • You are cleaning too much
  • New fish are not producing enough waste
  • Low pH levels

Ideally, try to keep pH levels in the tank to 7.0 and above. You could also be disposing of too much water too frequently, which also stalls the progress.

Make sure you do not disturb the tank more than is necessary and do not use chlorinated water as that creates uninhabitable conditions for bacteria.

My Nitrate Levels Aren’t Rising.

Here again, make sure you aren’t using chlorinated water or are cleaning your water too frequently. You may also be disposing of too much Nh3 and Nitrites, which means you are killing of nitrites, the source of nitrates.

My Fish Tank Won’t Cycle.

There can be multiple reasons why your aquarium won’t cycle. But the most common is the NH3 levels are too low.

Use a test kit to test the water parameters in the new tank and wait for NH3 to rise using some of the tips above.

Once it starts to rise, it should drop within three to five days, given ideal conditions.

Algae Blooms & Cloudy Aquarium Water

The nitrification process involves leaving your aquarium un-cleaned for extended periods, which can also cause algae to bloom.

Excess nutrients can lead to algae blooms; to solve this, do more small water changes each week to help remove any nutrients the algae might be feeding on.

Curbing algae growth can also be challenging if you have plants. If you have too much light, algae will take over. Make sure you give your plants no more than 10 hours of light a day to stop algae bloom.

Lastly, as long as you don’t over-fertilize your new fish aquarium, you won’t have to worry about algae. It

Final Thoughts

You will have to continually monitor your tank’s water chemistry even after the nitrogen levels reach 0, and you populate the aquarium.

Once you add more new fish to the tank, this process will repeat itself and continue forever.

Thanks for reading!

This article is part of a large series of articles to help you understand the benefits, myths, cost, and steps you need to take to set up a successful aquarium.

Below are links to the next and previous chapters in this series.

Now let’s get started on setting up your new aquarium.

Jack Dempsey
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