The Importance of Cycling Your Fish Tank and How to Do It

Water cycling is often overlooked, but it’s essential to give your fish a shot at happy, healthy lives.

May 23, 2024By Maya Keith
cycling your fish tank how to do it

In most cases, it’s best to set everything up before you bring a new pet home; fish are no exception. Instead of a crate or cat tree, however, your job is to make sure the water in their tank is safe.

This is referred to as “cycling”. While it can technically function without cycling, an uncycled tank is more likely to become unbalanced and negatively affect fish health.

In this guide, we explain why this is so important and how you can easily cycle your fish tank.

A Basic Rundown of the Nitrogen Cycle

personal fish tank
Image Credit: Emilia Murray, CC BY-SA 2.0 DEED, via flickr

The term “cycling” comes from the idea that you are establishing bacteria needed to maintain the nitrogen cycle in your fish tank. These beneficial bacteria will tackle the ammonia that comes from tank waste, ensuring a safe environment and limiting any tank cleaning you need to do.

The ecosystem within a fish tank relies on a biofilter and nitrifying bacteria to handle the ammonia load. These bacteria eat the ammonia and produce nitrite, which is then consumed by other bacteria and released as nitrate (which is far less toxic to fish).

The nitrate is then either consumed by live plants in the tank, removed through water changes, or (in very rare cases) converted to nitrogen gas.

A properly cycled tank will have nullified levels of ammonia and nitrate (meaning that they are consumed at a rate equal to their production). Because of this, your fish are far less likely to experience issues related to ammonia or nitrite poisoning.

What You Need to Cycle a Fish Tank

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Contrary to popular belief, you don’t need fish to cycle a fish tank.

Even first time fish owners should have everything you need to cycle a fish tank.

This includes everything you need to set up a tank, such as:

  • A source of ammonia (fish food works fine, but you can also use ammonia chloride)
  • Aquarium test strips (specifically for ammonia, nitrite, and nitrate levels)
  • Pumps, filter, substrate, decor (make sure you choose a quality filter)

It’s also important to use properly conditioned water for your aquarium. Regular tap water contains chlorine and chloramine, both of which will kill your beneficial bacteria and affect the pH of the water.

For optimal cycling, aim for a tank temperature between 83° and 86° Fahrenheit with a pH between 7.0 and 7.8. While these aren’t required for cycling, these parameters are most beneficial to the process. Anything too acidic will cause ammonium levels to rise, while basic water causes ammonium to convert to ammonia and further disrupts the cycle.

Cycling Your Fish Tank WITHOUT Fish

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Image Credit: Daniel Ramirez from Honolulu, USA, CC BY 2.0, via Wikimedia Commons

Whenever possible, cycle your tank without any fish in it. This not only decreases the variables affecting the establishment of the nitrogen cycle but limits any stress or casualties inflicted on the fish.

Set up all the components of the fish tank, including filling the tank with water, and make sure everything operates correctly. Your beneficial bacteria should have every necessary surface available to settle in on. Check the pH to make sure it isn’t too high or low (7.0 to 7.8).

Add Ammonia

Add your ammonia source. If you’re using fish food, feed as much as you would if the fish were in the tank; this helps set a basis for how much bacteria you may need in the future. As the food decays it will release ammonia that will feed and cultivate your beneficial bacteria.

After a day or two, check the ammonia levels of the water. The ideal number is around 3 ppm (parts per million), but anything up to 5 ppm should be fine.

Numbers below 3 ppm are unlikely to cultivate enough nitrite for a successful cycle, and you should add more food and time for decay before you move on. Numbers above 5 ppm merit a water change to attain a more manageable concentration.

Check Nitrite

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A sudden spike in nitrite levels can stress and kill even the hardiest fish.

Check the ammonia daily, aiming for steady levels around 3 ppm. Once this is achieved, test your nitrite levels. If they’re not detectable after a week, take more time and monitor your ammonia and pH. Otherwise, expect them to spike quickly as they feast on the ammonia.

Keep your ammonia levels up as well to ensure the nitrite does not starve, leading you to start the process again. Continue to monitor your parameters every 1 to 3 days, paying close attention to the ammonia and nitrite levels. Once the nitrite starts to dop, it’s time to check for nitrate.

Nitrate Levels

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Like shrimp, nitrifying bacteria work hard all day to keep your tank clean.

A drop in nitrite indicates that either the nitrifying bacteria are starting to chow down or your ammonia levels are too low to sustain the population. A test that shows the presence of nitrate is a good sign because it means the bacteria are present and doing their job. No nitrate means you need to bump up the ammonia.

Once you detect nitrate things start to slow down. Continue to make sure the bacteria have just enough to feed on, cutting back to about half as much fish food as before and checking water parameters every 1 to 3 days.

Once the ammonia and nitrite levels balance out, leaving the nitrate levels steady, you’ve finished cycling your tank and can add in your fish.

Adding Your Fish

It’s best to add fish one to 3 at a time, ensuring the bioload doesn’t overwhelm your newly settled bacteria. After they’ve been there for a day or 2, check the ammonia levels. If the cycle is still keeping them at 0, introduce more fish.

Continue this process until all of the fish are moved in and continue to monitor the ammonia levels periodically. After this, it’s up to you to ensure the tank is well-maintained.

Cycling Your Fish Tank WITH Fish

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If you must cycle a tank with fish, keep the initial population low.

Cycling your tank with fish is not recommended because it is more difficult and causes undue stress, but it’s unavoidable in certain cases. If you must introduce fish during the cycling process, make sure they’re hardy, low-maintenance species likely to survive the endeavor. We recommend you start with no more than 3 fish; too much waste will cause unmanageable ammonia levels in an uncycled tank.

Otherwise, cycling with fish follows the same steps of adding ammonia then monitoring water quality, but you’re actually feeding the fish instead of empty water. If possible, feed the fish less often or smaller amounts to ensure you don’t overload the tank.

Cycling with fish also requires that you change about 10 to 20 percent of the water twice a week to limit the bioload. Measure the ammonia, nitrite, and nitrate levels, and adjust your schedule as you hit each milestone.

Additional Considerations for Cycling Your Fish Tank

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Image Credit: FrozenAvatar, CC BY-SA 4.0, via Wikimedia Commons

Cycling your fish tank with plants is even more unpredictable than cycling with fish, and we strongly recommend against it. Understand that cycling with plants increases your chances of having to start over.

If possible, consider “seeding” your aquarium using a pre-established medium, like a portion of a filter or some substrate from a healthy tank of similar parameters. These materials can introduce a starter colony of beneficial bacteria and more quickly establish the nitrogen cycle in the new tank.

Whatever method you choose, make sure you approach the task with patience and understanding. It takes about 4 to 8 weeks to completely finish the nitrogen cycle, not counting any time for mistakes or fluctuations. Give yourself as much grace as possible, and understand the end results are well worth the effort.

Maya Keith
By Maya Keith

Maya is a lifelong animal lover. While she switched from studying veterinary medicine to English, she continues to help by fostering animals in her community. Her permanent residents include 3 dogs, 2 cats, 5 quail, 19 chickens, and a small colony of Madagascar Hissing Cockroaches.