How to Cycle a Fish Tank

Co-authored by Aaron Bernard

Last Updated: February 3, 2023 References Approved

The nitrogen cycle (also known as the nitration cycle) is the process that breaks toxic nitrogen waste products in an aquarium into less harmful components. For this cycle to develop, beneficial bacteria that feed on these waste products need to grow in the aquarium's filter system. Introducing fish to an aquarium without a healthy nitrogen cycle in place is a bad idea — buildup of waste chemicals can put major stress on the fish, even potentially killing them. Thus, cycling is something that every new aquarium owner needs to do to ensure the health and safety of his/her fish.^{[1]
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Method 1

Method 1 of 4:

Cycling with Fish

1
Set up your aquarium and filtration system. To start, you'll want your aquarium completely assembled and filled with everything you want in it, besides the fish. See our articles on setting up freshwater and marine aquariums for more information. Below is a brief checklist of things you'll want to do before getting started — this may not match all aquariums perfectly:
- Assemble the aquarium
- Add substrate
- Add water
- Add air stones, air pumps, etc.
- Add plants, rocks, etc.
- Add filtration system (and/or protein skimmer)
- Add heater
2
Introduce a small number of hardy fish to the tank. Your goal in this cycling process is to populate the tank with fish that produce waste but can survive the initial high levels of toxins long enough for the beneficial waste-processing bacteria to grow. Thus, you'll want to pick a variety that is known for being a good cycling fish and start with a small number. Later, once the bacteria have grown, you can slowly add more fish of different types. Below are just a few good choices for cycling fish:^{[2]
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- White Clouds
- Zebra Danios
- Cherry or Tiger Barbs
- Pseudotrophius Zebra
- Banded Gouramis
- X-ray Tetras
- Pupfish
- Most minnows
- Most guppies
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3
Feed fish sparingly. When cycling an aquarium with your fish, it's very important not to overfeed them. Though different fish may have different dietary needs, a good rule of thumb is to give food once every other day. Only offer a moderate-sized meal — you don't want any extra food left over when the fish are done eating. This is done for two reasons:
- Fish who eat more produce more waste, which can cause the levels of toxins in the tank to rise before the bacteria have a chance to colonize the aquarium.
- Leftover food will eventually rot, producing toxins on its own.
4
Perform frequent water changes. While you're waiting for your tank to cycle, every few days, replace about 10-25% of the tank's water. As with the reduced feeding schedule described above, this is another way of ensuring that toxin levels don't get too high before the bacteria has a chance to grow. If you have a saltwater tank, don't forget to add a suitable amount of marine salt every time you change the water to keep the tank at a proper salinity.^{[3]
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Expert Source

Aaron Bernard
Fish & Aquarium Specialist
Expert Interview. 17 July 2020.}
- Don't use chlorinated water — this can kill the bacteria in the tank, forcing the cycle to start over. If using tap water, make sure to treat it with an appropriate dechlorinator or water conditioner before adding it to your aquarium. If using bottled water, make sure to buy distilled water, as "purified" or "drinking" water may have minerals added for taste that may be harmful to fish.
- Be ready to perform water changes much more frequently if you start to see signs of serious ammonia stress in your fish (more information below in the "Solving Common Problems" section.) However, try to avoid stressing the fish by exposing them to big changes in water chemistry or temperature.
5
Use test kits to monitor toxin levels. When you add fish to your tank, the levels of toxic chemicals known as ammonia and nitrites will quickly rise as the fish release waste into the water. As beneficial bacteria begin to grow in response to these chemicals, their levels will gradually drop to near-zero, at which point it's safe to add more fish. To monitor these chemicals, you can use commercially-available test kits, which are usually sold at the same places fish and aquariums are. Testing daily is ideal, but you can sometimes get away with testing every few days.
- You will want to keep ammonia levels below 0.5 mg/L and nitrite below 1 mg/L throughout the cycling process (ideally, they should be less than half of these values.) If these chemicals start to approach unsafe levels, increase the frequency of your water changes.
- The cycling process is complete when both ammonia and nitrite levels drop so low that they are undetectable. For practical purposes, this is often referred to as "zero," though this isn't technically accurate.
- As an alternative, you can take water samples to the pet store where you bought your fish or aquarium. Most will offer cheap testing services (some even do it for free!)^{[4]
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6
Add additional fish gradually once toxin levels are near-zero. The cycling process typically takes about six to eight weeks. Once ammonia and nitrite levels are so low that they aren't showing up on your tests, you may add more fish. However, you'll want to do this gradually, introducing just one or two new fish at once. Adding just a few fish at a time keeps the increased amount of ammonia and nitrites in the tank from each new addition well within the ability of the bacteria to control.
- After each addition of new fish, wait at least a week or so, then test the water once more. If ammonia and nitrite levels are still low, you may add your next few fish.

Method 2

Method 2 of 4:

Performing a "Fishless" Cycling

1
Assemble and prepare your tank. Here, we're starting with a fully-assembled tank, but without the fish. We won't add the fish until the entire cycle is completed. Instead, we'll add biological waste manually as we monitor the water level and wait for the cycle to complete.
- This method requires a lot of patience, as it requires you to wait for the organic material you add to your tank to decay and start producing toxic waste products. However, it's often considered a more "humane" option as it doesn't expose fish to ammonia and nitrites like the method above does.^{[5]
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2
Add a sprinkling of fish flakes. To begin, drop just a few flakes of fish food into your tank — about as much as you'd use to feed your fish will do. Now, simply wait. Over the next few days, the flakes will begin to decay and release waste products (including ammonia) into the water.
3
Test your water for ammonia in a few days. Use a test kit (or bring a water sample to your local pet store) to test your water for ammonia levels. You want to have a level of at least three parts per million (ppm). If you don't have enough ammonia in your water, add more flakes and wait for them to decay before testing again.
4
Try to keep the ammonia level at about three ppm. Continue to test your water every other day for ammonia levels. As beneficial bacteria begins to grow in your aquarium, it will start to consume the ammonia, reducing the ammonia levels. Replenish them by adding fish flakes whenever the ammonia level falls below three ppm.
5
Start testing for nitrites, after a week. As the bacteria start consuming ammonia, they'll begin to produce nitrites, the intermediate type of chemical in the nitrate cycle (which is less toxic than ammonia but still harmful to fish). Start testing for nitrites after a week or so — again, you can use a commercial test kit or take water samples to a pet shop to do this.
- Once you detect nitrites, you'll know the cycle has started. At this point, you'll continue adding ammonia as you have before.
6
Wait for a sudden drop in nitrites and a rise in nitrates. As you feed the bacteria in the tank ammonia, the nitrite levels will continue to rise. Eventually, however, enough beneficial bacteria will grow to convert the nitrites into nitrates, the final type of chemical in the nitrate cycle (and one that isn't harmful to fish.) When this happens, you'll know the cycle is nearing completion.
- You can detect this final phase of the cycle by either testing for nitrites (in which case you're looking for a sudden drop), nitrates (in which case you're looking for a sudden spike from a base level of zero), or both.
7
Add fish gradually when ammonia and nitrite levels are near zero. After about six to eight weeks, the ammonia and nitrite levels should decrease to a level that's so low you can no longer detect them, while nitrate levels should plateau. At this point, it's safe to add your fish.^{[6]
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- However, as in the method above, you'll want to add your fish gradually. Don't add more than a few small fish at a time and wait at least a week or two before introducing your next batch of fish.
- Consider cleaning the substrate with a siphon hose before adding fish, particularly if you had to add a lot of food. Decaying food or plant matter can become a ticking time bomb. If it gets trapped in the gravel, the ammonia won't enter the water, but if something upsets it, it could release a fair amount of ammonia rather quickly.

Method 3

Method 3 of 4:

Speeding Up the Cycling Process

1
Add filter media from a mature tank. Since cycling a tank can easily take up to six or eight weeks, aquarium owners have long been looking for ways to shorten this process. One proven way to do this is to introduce bacteria from a tank that's already been cycled to the new tank. Since you don't have to wait for the bacteria in your tank to start growing naturally, your tank should cycle quicker than it otherwise would. One great source of bacteria is a tank's filter — simply switch the filter media from the established tank to the new tank for a potential boost.
- Try to use filter media from a tank that's a similar size and has a similar amount of fish. Mis-matching your filters (like, for instance, using a filter from a tank with just a few fish in it to cycle a tank with a greater number of fish) can leave you with greater loads of ammonia than the bacteria are able to process right away.
2
Add gravel from a mature tank. In the same way as filter media can allow you to "transplant" bacteria from an established tank to a new one, an established tank's substrate (the gravelly material at the bottom) can give you the same effect. Simply add a few scoops of substrate on top of the tank's existing substrate to get the benefit.
3
Have live plants in the aquarium. Living plants (as opposed to fake plastic ones) typically speed up the nitrogen cycle, especially if they're introduced from a mature tank. Not only can plants carry beneficial bacteria (just like the substances above), but they also pull ammonia out of the water directly to use in a biological process called protein synthesis.
- Fast-growing plant varieties (like, for instance, Vallisneria and Hygrophila) tend to absorb the most ammonia. Floating plants also generally work well.
4
Beware of the risk of cross-contamination. One possible downside of using filter media or substrate from one tank to transfer beneficial bacteria to another is that it's also possible to unwittingly transfer other organisms. Many parasites, invertebrates, and assorted microorganisms can be transferred in this way, so be aware of this possibility in advance and never transfer material from a tank that's known to be contaminated with harmful organisms.
- Pests that can be transferred this way include snails, harmful algae, and parasites like ich and velvet.
5
Add small amounts of salt to freshwater tanks. If you have a freshwater tank, adding a very small amount of salt can help your fish stay healthy when toxin levels are highest at the start of the cycling process. It does this by reducing the toxicity of nitrite, the intermediate chemical in the nitrate cycle. However, you'll want to use only 0.4 ounces per gallon of water at most — any more can be very stressful for freshwater fish.
- Be sure to use certified aquarium salt — table salt isn't formulated for your tank and may hurt your fish.

Method 4

Method 4 of 4:

Solving Common Problems

1
Treat ammonia stress during cycling with frequent water changes. Ammonia stress (the dangerous symptoms that fish get when ammonia levels get too high) is always a risk during the cycling process. If they aren't dealt with quickly, these symptoms can eventually become deadly for the fish. If you see the symptoms below, lower the ammonia levels by changing the water more frequently and changing a greater portion of the water each time:^{[7]
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- Lethargy/lack of movement (even when food is added)
- Refusing to leave the bottom of the tank
- Gasping for air at the surface of the water
- Inflamed eyes, gills, and/or anus.
2
Consider ammonia neutralizers if you run into toxicity problems. There are two kinds: remover and detoxifier. Most pet shops and aquarium stores will sell chemicals specifically designed to neutralize ammonia in aquariums. Though these can be useful if the ammonia level becomes so high that it begins to harm the fish, they are more useful in starting a new tank as they permit to skip some water changes, shortening the time needed to cycle a new tank.
- Some people believe that ammonia removers can be harmful in the long-run. ^{[8]
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  Research source} This may be due to a misunderstanding of the process of detoxifying. In a tank, toxic ammonia (gas NH3) is in reversible equilibrium with not-so-toxic ionized ammonia (NH4+). Most of the detoxifier products convert toxic ammonia into the form that isn't so harmful to fish. However, after 24 to 48 hours, they will release the ammonia. That's why these products should be used:
  - as long as the useful bacteria is not yet established AND,
  - from time to time make a partial water change (as per manufacturers' instructions) to remove some of the ammonia accumulated^{[9]
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  - even if is not specified, dose the detoxifier for the whole tank, not just for the new added (replaced) water, as the already bonded ammonia in the tank will be released soon (after 24-48 hours from previous dose).
- Changing 50% of the water (or more) generally prolongs the time required to cycle the tank (or even stop the cycle) just because the useful bacteria will be temporary inhibited and will need time to adapt to new pH.^{[10]
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  Book: "Nitrification and Denitrification in the Activated Sludge Process"/Michael H. Gerardi}. For this reason, some recommend a pH change of less than 0.2-0.3 per day. Suppose you have 7.8 pH in tank, replacing 25% with water of pH=7 will bring the final pH to 7.6.
- Useful bacteria transforms only ionized (non-toxic) form of ammonia, so they benefit from these products also.^{[11]
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3
Use goldfish only for cycling all-goldfish tanks. Though they're often thought of as the quintessential aquarium fish, goldfish are actually not recommended for cycling a tank. The problem with goldfish stems from the facts that they have different care requirements than the types of tropical fish that are most common in aquariums today. Thus, cycling a tank with goldfish and then adjusting the tank to accommodate tropical fish can cause at least some of the bacteria to die from the higher heat and different water conditions.^{[12]
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Research source} This stresses the goldfish, the bacteria, and the tropical fish — not a recipe for a healthy tank.
- In addition, modern goldfish are somewhat susceptible to diseases that can easily spread through the entire aquarium.^{[13]
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- You won't want to cycle any aquarium with so-called "feeder" goldfish, which are poorly-cared for by breeders and sellers and are extra-susceptible to disease.^{[14]
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Expert Q&A
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Question

What's the point of cycling a tank?

Aaron Bernard
Fish & Aquarium Specialist
Aaron Bernard is an Aquarium Specialist and the Owner of Limited Edition Corals in Phoenix, Arizona. With over ten years of experience, Aaron specializes in aquarium maintenance, custom design, manufacture, installation, and moving. Aaron holds a BS in Biochemistry from Arizona State University, where he studied the effects of climate change on coral reefs and began to propagate his corals.

Aaron Bernard

Fish & Aquarium Specialist

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You're basically establishing the nitrogen cycle in the tank. Ammonia is toxic and it will kill fish. It'll actually kill anything in the water. So you need to get rid of it and reestablish the nitrogen levels. That's the point of cycling the tank.
Question

Do you need fish to cycle a tank?

Aaron Bernard
Fish & Aquarium Specialist
Aaron Bernard is an Aquarium Specialist and the Owner of Limited Edition Corals in Phoenix, Arizona. With over ten years of experience, Aaron specializes in aquarium maintenance, custom design, manufacture, installation, and moving. Aaron holds a BS in Biochemistry from Arizona State University, where he studied the effects of climate change on coral reefs and began to propagate his corals.

Aaron Bernard

Fish & Aquarium Specialist

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No, you can run a fishless cycle. A lot of people actually prefer fishless cycles since you don't need to worry about harming your fish in the process.
Question

Why are fishless cycles faster?

Aaron Bernard
Fish & Aquarium Specialist
Aaron Bernard is an Aquarium Specialist and the Owner of Limited Edition Corals in Phoenix, Arizona. With over ten years of experience, Aaron specializes in aquarium maintenance, custom design, manufacture, installation, and moving. Aaron holds a BS in Biochemistry from Arizona State University, where he studied the effects of climate change on coral reefs and began to propagate his corals.

Aaron Bernard

Fish & Aquarium Specialist

Expert Answer

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You can intentionally put ammonia in the tank without worrying about killing your fish. That's what moves the process along so fast.

Warnings

Using large chunks of food or organic material to cycle (release ammonia) may result in a bacterial bloom and unpleasant smell. Food can also mold underwater, sickening your fish and allowing a colony of mold to grow into your substrate.

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Nitrates over 40 ppm and ammonia/nitrites over 4 ppm means you need to do a slight water change, as these can be detrimental to your healthy bacteria you're trying to grow.

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