A Simple Guide On How To Cycle Your Fish Tank

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If you have a new fish tank, you must cycle it before you can add any fish. Cycling the tank allows the beneficial bacteria that you need for the nitrogen cycle to become established in the filter media. That ensures that any toxic ammonia and nitrites have been neutralized so that the water is safe for your new fishy friends. 

But how long to cycle a tank? And can the use of aquarium bacteria supplements cut fish tank cycling time?

In this beginner’s guide, we explain how to cycle a fish tank the right way, including how to cycle a tank with fish and how to do a fishless cycle.

What Is The Nitrogen Cycle And Why Is It So Important?

Nitrogen Cycle Diagram

In the natural environment, the Nitrogen Cycle is a natural full cycle where nitrogen gas goes from air to plant to animal to bacteria and then back to air.

However, things are different in the enclosed environment of your fish tank. Here, the Nitrogen Cycle is more of a biochemical process than a cycle. 

Briefly, here’s how that process works:


During the first stage of the cycle, fish waste and other decomposing organic matter produce ammonia. This highly toxic gas will kill your fish and other aquarium inhabitants if it’s not removed from the water. 

The ammonia is oxidized (processed) by the Nitrogen-fixing bacteria that live in your filtration system’s biological filter media and on surfaces within the tank. Through that process, the ammonia is converted into nitrite.


Nitrite is also highly poisonous to fish and can result in mass fish kills if the Nitrogen Cycle is not working correctly.

Other species of bacteria in your filter media convert nitrite into nitrate. 


Nitrate is much less harmless to fish, although you do need to keep levels below 20ppm in your tank to keep the environment healthy for your fish.

In A Nutshell . . . 

So, that’s really all there is to the Nitrogen Cycle!

Fish waste, uneaten food, plant debris, and other organic matter decomposes, releasing ammonia into the water. Beneficial bacteria convert the ammonia to nitrites, and then other bacteria process the nitrites into less harmful nitrates.

You carry out weekly partial water changes to remove the nitrates from the water, keeping the tank safe and healthy for your fish.

What Happens If You Don’t Cycle Your Aquarium?

If you don’t cycle your aquarium before adding fish, the filter media won’t contain the numbers of beneficial, nitrogen-fixing bacteria that are necessary to process the ammonia and nitrites into nitrates.

That means when your fish begin producing waste, there’ll be nothing to remove the ammonia from the water. An ammonia spike will occur, quickly killing all your fish.

How Long Does It Take To Cycle Your Aquarium?

In a fish tank, it takes time for the Nitrogen Cycle to become fully established. As a ballpark figure, it can take anywhere between a few weeks and a few months for a living aquarium to be fully cycled.

A green beautiful planted tropical freshwater aquarium with fishes

During that time, it’s crucial that you only introduce a few small fish at a time so as not to overload the biological filter. 

Throughout the cycling process, test your aquarium water every couple of days to monitor the levels of ammonia, nitrites, and nitrates in the water. Once ammonia and nitrite levels are zero and nitrates are 20ppm (parts per million) or below, you can add a few small fish.

How To Cycle Your Aquarium The Easy Way

The so-called fishless cycle is the simplest way to cycle your aquarium, so we’re going to begin by giving you an overview of how that process works.

What You’ll Need

Here’s what you’ll need for a fishless cycle:

  • Dechlorinator
  • Fish food
  • Aquarium water testing kit

Set up your fish tank, filling it with dechlorinated tap water. You must use dechlorinated water, as the chlorine that tap water contains will harm your fish. 

It can be helpful to create a chart so that you can keep a timeline showing the changes in the levels of ammonia, nitrites, and nitrate in the water during the cycling process.

Stage 1 – How To Start The Cycle

To begin the Nitrogen Cycle, the water must contain some ammonia. But you don’t have any fish in the tank, so what can you do?

As you know, decomposing fish food releases ammonia, which is why you should never overfeed your fish. So, add a few fish food flakes to the fish tank every 12 hours for a few days. Within a few days, the rotting food will start to produce ammonia.

Alternatively, for an instant kickstart, you could add a few drops of pure ammonia to the water. 

Stage 2 – Test The Water For Ammonia

To know whether there’s enough ammonia in the water to start the cycle, you’ll need to test the tank every few days. Ideally, you want the ammonia level to be at least 3ppm. If necessary, add more fish food and leave them to decompose.

Test the water on alternate days and try to keep the ammonia levels at 3ppm. The Nitrosomonas bacteria that process ammonia will quickly begin to grow and do their work. Whenever the ammonia levels fall below 3ppm, add more flakes.

To that for a week.

Stage 3 – Nitrite Testing

Continue testing the water, but this time you’re looking for nitrites. Once nitrites are present in your tank, you know that the cycle has begun.

You still need to provide fuel for the Nitrosomonas, so keep adding fish food to the water.

Stage 4 – Nitrate Testing

After a few weeks, you should see the levels or nitrites begin to fall. When that happens, you know that it’s time to begin testing the water for nitrates.

Once you can detect nitrates in the water, you know that the Nitrogen Cycle is almost complete. Ideally, you want the nitrate levels at 20ppm or less. If the levels are higher than that, you’ll need to do some partial water changes to get that number down.

Stage 5 – Add Your Fish

Once you’re sure that there’s no detectable ammonia or nitrite in the water, you can add a few fish.

Little happy boy holding a plastic bag with new fishes he bought at the zoo store for his home room aquarium feeding and taking care of pets

And I mean a few fish only! Don’t throw a whole load of critters into your tank, to begin with. That’s just going to overload the biological filter and could create problems with the water quality.

Before you add your new fishy friends, I recommend that you use an aquarium vacuum to remove any decaying fish food that’s trapped in the substrate. Pockets of rotting food can create dangerous ammonia bombs that may be released into the tank, potentially harming your fish and upsetting the delicate balance of the new tank.

Managing Your New Tank

Over the next few weeks, while the tank is becoming established, it’s crucial that you manage the water quality carefully.

Detail of man's hand pruning the plants of his aquarium with scissors

Don’t overfeed your fish, and carry out partial water changes every few days to keep nitrate levels down. Keep testing the water to make sure that ammonia and nitrites are at zero.

Fish-In Cycling

I will preface this section of our guide by saying that the fish-in method of cycling a fish tank is not our preferred way of doing things. 

Fish-in cycling uses small, low-value fish that are regarded as expendable. In fact, some hobbyists call these unfortunate creatures “suicide fish.” That’s because the fish will be exposed to high levels of ammonia and nitrites and often don’t survive.

That said, some fish species do seem able to withstand the shock of such unsuitable water conditions and will survive.

What Are Good “Cycling Fish?”

Here are some species of fish that are reputedly able to survive the cycling process in a brand-new setup. Even so, you should still expect a few fatalities.

Ideally, you want to add one to two fish per 10 gallons of water. Don’t add too many fish, or that will cause an ammonia spike that will most likely kill off all the fish.

Stage 1 – Add A Few Hardy Fish

With this method of cycling, you’re going to introduce ammonia to the tank through fish waste and fish food. The fish you choose must be able to survive the conditions for long enough to enable the beneficial bacteria to colonize the filter media and tank surfaces.

Stage 2 – Feed Your Fish

Be very careful that you don’t overfeed the fish. Feed the fish once every two days, offering only what the fish will eat within a couple of minutes.

Basically, the more the fish consume, the more waste they will produce. That waste boosts the levels of toxins in the tank before there are large enough colonies of bacteria to cope. So, by feeding the fish sparingly, you’re helping to keep toxins in the water at manageable levels.

Stage 3 – Perform Partial Water Changes

Young man changing water in aquarium using siphon.

Since the fish in your new aquarium are being bombarded by lots of potentially lethal ammonia and nitrate, you must carry out partial water changes regularly to keep the toxin levels down.

Ideally, you want to change 10 to 25% of the water every two or three days. It’s a tricky balance, as you don’t want to take out too much ammonia and nitrite, or your beneficial bacteria will have nothing to eat. 

Every time you add fresh tap water to the tank, be sure to add a de-chlorinator to the water. If you put water containing chlorine or chloramine into the tank, you’ll kill the beneficial bacteria and stop the cycling process in its tracks.

Stage 4 – Test The Water

Test the aquarium water regularly to keep an eye on the ammonia and nitrite levels in the tank. When nitrite levels drop to zero and you begin to find that there are nitrates in the water, you’ll know that the cycling process is complete.

Ideally, you want to test the water every day, although testing every two to three days is fine.

Stage 5 – Add More Fish

Once the levels of ammonia and nitrite in the tank have reached zero, you can begin to add a few more fish.

Don’t go crazy and put a whole bunch of fish in the aquarium! Introduce one or two fish each time. Wait a week or so, and then test the water again. If the levels of ammonia and nitrite are still very low, you can add more fish.

It’s critical that this process is taken slowly. If you add too many fish, the levels of ammonia and nitrite will shoot up, stressing your fish and potentially causing outbreaks of disease.

Cycling With Plants

close up image of landscape nature style aquarium tank with a variety of red stem aquatic plants inside.

Live plants make an excellent addition to any freshwater fish tank. Plants not only look beautiful, but they utilize excess ammonia and nitrates as fertilizers, helping to balance the new aquarium, especially if you decide to use fish to cycle it.

You’ll need to have your aquarium lighting on for between eight and 10 hours every day to ensure that the plants get sufficient light for photosynthesis. 

Can I Speed Up The Cycling Time?

The good news for those of you who can’t wait six to eight weeks or longer for your new aquarium to cycle is that there are a few ways of speeding up the process.

All these methods involve “seeding” your new tank with material from an existing setup.

Use Filter Media From A Mature Aquarium

Aquarium filter media in bowl over white backgdround

If you have access to an existing tank, you could use some of the filter media from that. The filter media in an established aquarium will already contain nitrifying bacteria, so you won’t need to wait for the colonies to proliferate naturally. 

Tip: If you’re using the fish-in cycling method, try to use filter media from a tank of a similar size that contains a similar number of fish.

Use Substrate From An Established Tank

The substrate in your tank also provides a surface for beneficial bacteria and acts as part of your aquarium’s filtration system. So, if you have access to an established tank, especially if it uses an undergravel filtration system, there will be a fair amount of bacteria living within the substrate.

Close up shot of hands putting plants on the low water aquarium.

If your new tank has an undergravel filter, place a cupful of substrate from the existing tank across the gravel. Alternatively, hang a mesh bag full of the old substrate in your new aquarium’s filter system.

“Season” Your New Filter Media

Take the filter that you’ll be using in your new tank and fix it next to the filter in an established tank. Allow the two systems to run alongside each other for a week or so to allow the bacteria from the existing system to colonize the new filter.

The new filter will now be “seasoned” and ready to use in your new tank.

Use Living Plants From An Existing Tank

hands of aquarist planting water plant echinodorus in new aquarium

Living plants carry colonies of bacteria on their leaves, so adding a few plants from a mature tank can help to accelerate the cycling process in a new setup.

Also, thanks to a process called protein synthesis, living plants can help to moderate the levels of ammonia in the tank. 

BUT . . . 

There is a very large “BUT” that’s associated with using media and plants from existing tanks. 

Before adding anything to a new tank from an established aquarium, you must be 100% certain that you won’t be bringing across anything nasty such as disease-causing bacteria or parasites.

Check that all the fish and plants are healthy, and never use anything from a tank that’s recently suffered a disease outbreak or fish mortality.

Problems With Using Seeding Material From Existing Tanks

The main issue with using seeding material from an existing tank is that the water parameters of the two aquariums must be very similar for the process to work.

The pH and KH levels must be similar in both tanks, as bacteria can’t tolerate dramatic variations in water chemistry. Also, your new fish will bring some bacteria with them, so the bacteria that you finish up with might not be what you started out with.

What About Over-The-Counter Bacterial Supplements?

Although many of the over-the-counter bacterial supplements that you see for sale in your local fish store can be very effective at boosting levels of beneficial bacteria, you shouldn’t rely solely on these to cycle your new aquarium. 

You also need to check that the product is still within the expiry date so that the bacteria contained in the supplement are still viable.

Nitrogen Cycle Problems And Solutions

No matter how careful and diligent you are, there are a few common problems that can happen when you’re cycling a new tank.

Ammonia Poisoning

If ammonia levels get too high, the effect on your fish can be devastating. So, if you’re using the fish-in cycling method, you need to be aware of the signs of ammonia poisoning.

Symptoms of ammonia poisoning include:

  • Not eating
  • Inactivity
  • Sitting on the bottom of the tank
  • Hanging at the water surface, gasping for air
  • Inflamed or red anus, gills, or eyes
  • Red streaks in the finnage

To save your fish, you must deal with incidents of ammonia poisoning immediately. Carry out partial water changes spaced out over a few days, and test the water to make sure that ammonia and nitrite levels are as close to zero as possible.

Tank Doesn’t Begin Cycling

Usually, the ammonia levels in the tank should begin to rise by day three. If by day five there’s still no ammonia in your test readings, it’s possible that your test kit is faulty. However, if you get the same result with a fresh test kit, your aquarium might not be cycling.

Generally, the reason for that is that you’re not providing a source of ammonia or not enough ammonia for the bacteria. Solve the problem by adding more ammonia. It’s also possible that your live plants are scoffing all the ammonia before the bacteria get to it. So, remove a few plants.

Test the water again after a few days, and if you still don’t see any ammonia or nitrites, add some more ammonia.

Ammonia Levels Are Not Falling (Fishless Cycling Method)

If the ammonia levels aren’t dropping and you’re using the fishless cycling method, there are three  main reasons for that:

  • pH is too low
  • You’re using water that contains chlorine or chloramine
  • You’ve cleaned the tank too much

If the pH is less than 7.0, ammonia will be present in the tank as ammonium, which the nitrifying bacteria cannot process. Use a pH kit to increase the pH, and the problem should sort itself out.

Water containing chlorine will kill the beneficial bacteria in the filter. So, for a quick-fix, always add dechlorinator to the tap water you’re using for partial water changes.

Go steady when cleaning the tank. The beneficial bacteria you need for the cycling process live in the filter media, substrate, and decorations in your tank. Too much cleaning will remove the bacteria before it has a chance to establish sufficiently large colonies.

Nitrate Levels Are Not Rising

If your nitrate levels are permanently at zero, probably that’s because you’re killing the bacteria before they’re established properly.

So, again, don’t use chlorinated water or get too busy cleaning your tank.

Algae Bloom

Algae bloom is a common problem when cycling a new tank. 

Deal with the problem by reducing the amount of light in the tank, avoiding using plant fertilizer, and using a CO2 injector.

If you have live plants in the tank, reduce the amount of light they get to ten hours per day, no more.

Final Thoughts

There’s no quick-fix or silver bullet to instantly cycle your new fish tank. 

Unfortunately, nature can’t be hurried, and you need to be patient. Although the fish-in method of cycling your new tank might be quicker than the fishless technique, we recommend that you go fishless from a welfare point of view. 

You can use material from an existing, healthy tank to accelerate the cycling process slightly, but whichever method you go for, be sure to test your aquarium water every day or so with an aquarium test kit and monitor the levels of ammonia and nitrites carefully.

I hope you enjoyed this guide and found it helpful. If you did, please share!

Alison Page has been an avid fish keeper for over 35 years and has owned many different species of freshwater tropical fish including bettas. Currently Alison has two large freshwater tanks. The first tank has two huge fancy goldfish who are almost ten years old and still looking as good as ever. In the other, she has a happy community of tiger barbs, green tiger barbs, corydoras catfish, platys, and mollies.

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