how to cycle a fish tank - the nitrogen cycle

How to Cycle a Fish Tank: The Nitrogen Cycle

If you want to keep an aquarium with happy and healthy fish, you’re going to need to understand two important concepts. First, you need to know how to cycle a fish tank. This is how you prep your tank for your future finned friends. 

Secondly, you’re going to need to understand the aquarium nitrogen cycle. It is critical for keeping your water parameters balanced. This doesn’t mean pristine water necessarily though. Fish in the wild do not live in pure, crystal clear water with no other contaminants. Life flourishes when there’s a whole ecosystem of microorganisms, plants, and animals that are in balance with one another. That’s our goal with the nitrogen cycle.

If you’re feeling intimidated, don’t worry. In this guide, you’ll learn about the nitrogen cycle, how to cycle a fish tank, equipment needed to cycle a fish tank, and common  troubleshooting issues.

Table of Contents

Recommended for Cycling your Tank:

What is the Nitrogen Cycle? 

The nitrogen cycle goes by a lot of names such as the biological Cycle, nitrification process, or new tank syndrome. They all reference the same underlying concept – the nitrogen cycle.

Why does this cycle have so many names? why is it so important?

When your fish go to the bathroom in your tank, their waste releases toxic ammonia into the water. If  your fish were allowed to go to the bathroom indefinitely, their tank would get pretty gross quickly. Left unchecked, this will eventually kill your fish. 

The Nitrogen Cycle helps your fish avoid this sad death. In the nitrogen cycle, beneficial bacteria establishes itself in your tank and filter media (for example, bio balls). This helpful bacteria then converts ammonia (toxic) to nitrite (toxic), and then nitrite to nitrates (non-toxic). You are recycling nitrogen from one form to another.

Essentially, your fish’s waste converts from dangerous to not-dangerous by the work of the beneficial bacteria.

Nitrates are a good outcome of the nitrogen cycle. However, you can always have too much of a good thing. If you allow the nitrate levels to get too high, it will support algae taking over your tank and will cause a decrease in the appetites of your fish. That’s where water changes come into the picture.

How long does the nitrogen cycle take?

This is a million dollar question! The nitrogen cycle can take anywhere from 2 weeks to 2 months. There are tricks to speeding it up but the most important thing is to not jump ahead and add fish before your tank is ready.,

Cycling your tank is required for establishing your new aquarium. Otherwise, you will kill all the fish you add.

During cycling, you will need to monitor the tank conditions at least every other day. The easiest way to do this is using an aquarium test kit that looks at ammonia, nitrites, nitrates, and pH levels.

At the beginning, ammonia levels will rise. When nitrites start to take over, the ammonia will drop. Nitrate won’t appear on your test until you reach significant levels of it. Once it crosses the threshold, your nitrite levels will fall.

When you can no longer detect nitrites, your tank is prepped and ready for the first fish.

What happens if you don’t cycle your aquarium?

The easiest way to answer this question is to imagine a human living in its own waste. It is an unhealthy situation and at minimum, it will cause sickness. Depending on the conditions, it could easily cause death. This is the situation of adding a fish to an uncycled tank. 

Once you add the fish (a source of ammonia) to the tank, the tank will begin the cycling process. The question is then – will your fish survive the experience?

Stages of the Nitrogen Cycle

Knowledge is power so let’s learn more about the three stages of the nitrogen cycle:

  • Stage 1: Ammonia
  • Stage 2: Nitrites
  • Stage 3: Nitrates

Stage 1: Ammonia

Ammonia enters your fish tank through fish waste or uneaten food.

It will produce ionized ammonium (NH4) if the pH is below 7. If the pH is above 7, it will produce ammonia (NH3).

Ammonia will continue to build up in the tank until the bacteria that eats it start to form. You will see this marked by higher ammonia levels on your water tests.

When the good bacteria begins to form, your tank may get cloudy.

To graduate from this phase, you’re looking for an ammonia spike and subsequent quick decline. When you see this, you’re entering the second phase – nitrites.

Stage 2: Nitrites

Nitrites levels will rise as ammonia simultaneously declines. Bacteria called Nitrosomonas will oxidize the ammonia, turning it into nitrite. Nitrite is still very toxic to your fish so it is not safe to add fish when you see increasing levels of nitrites. Nitrate is a byproduct of this process.  

Similar to the first stage (ammonia), nitrite will continue to build up until a colony of bacteria forms to dispose of them. 

Nitrite levels will likely start to rise at the end of the first week or during the second week.

Stage 3: Nitrates

Congratulations, you have reached the final phase! Once your toxic nitrite reaches a certain level, a bacteria called Nitrobacter will develop. Nitrobacter’s job is to convert the nitrites into nitrates. 

When the levels of nitrite and ammonia reach 0ppm (parts per million), your tank has been officially cycled and you can safely add fish. Nitrates will be doing their job.

However, this doesn’t mean your work is done with the nitrogen cycle. While nitrates aren’t toxic in low concentrations, they can become toxic if they reach above 20ppm for certain species. This is where weekly water parameter testing comes into play. 

To keep your nitrates in check, two methods are commonly used: water changes and plants. First, perform frequent, partial water changes (20-50% every 1-4 weeks). This will remove nitrates and dissolved organic compounds (DOCs) and fish waste, keeping your tank inhabitants healthy.

For a freshwater fish tank, adding aquarium plants will help keep nitrates under control. Example plants include Amazon Swords, Java Moss, Anubias, and Java Fern.

For saltwater tanks, deep sand beds can have pockets without oxygen where beneficial bacteria break down nitrates into nitrogen gas.

How to Cycle a Fish Tank

There are three methods to cycle a fish tank: with fish, fishless, and with plants.

How to Cycle a fish tank with fish (not recommended)

This method is not preferred. You’ll directly expose your fish to ammonia, likely resulting in the death of the first few members of your tank. Another likely scenario is that you will stress your tropIcal fish out and then they will get sick.

Some species are hardy and might be able to pull through. If you’ve made the beginner’s mistake of buying your aquarium and fish on the same day, this may be your only option.

Step 1: Introducing A Small Number Of Hardy Fish

Your tank needs creatures that produce waste, and therefore ammonia, in order to get the nitrogen cycle going. You will want to select fish species that have a chance at surviving high levels of ammonia and nitrite long enough to allow for the beneficial bacteria to grow.

cycling a fish tank using a single fish

Aim for approximately 1-2 starter fish per 10 gallons of water. So in a 20-gallon tank, only add 2-4 fish. Too many fish will lead to excess waste which will spike your ammonia and kill all the tank inhabitants. Here are a few options for cycling fish:

  • Cherry or Tiger Barbs
  • Banded Gourmis
  • Most Minnows
  • Most Guppies
  • Zebra Danios

Step 2: Feed the fish

Feed your fish but do not overfeed them. When you’re cycling your tank, you will feed your fish less frequently than an established tank. As a general rule of thumb, feed your fish moderate meals only once every 2 days

Why this lean feeding strategy?

Fish that eat a lot produce more waste. More waste means more ammonia. This means more toxic materials in your tank before the good bacteria is able to colonize and stabilize your aquarium.

Additionally, any additional food that your fish don’t eat will decay, producing more ammonia and toxins, making a deadly cycle even more deadly. 

Step 3: Water Changes

Regular water changes are how you keep your fish alive during this process. 

Your fish are being exposed to potentially lethal amounts of ammonia and nitrate daily. Water changes physically remove some of these toxins, keeping levels low enough so your fish can stick around.  

For water change frequency, target a 10-25% water change every 2-3 days. 

You don’t want to remove too much water because you will remove the ammonia and nitrite that beneficial bacteria needs to feed on. This will jeopardize the beneficial bacteria’s ability to colonize.

Make sure you add de-chlorinator to the new water. Chlorine will kill the beneficial bacteria and ruin the cycling process.

Step 4: Testing toxin levels

A test kit is required during this process so you can monitor the ammonia and nitrite levels in your aquarium.

pH test in aquarium cycling

Again, you will initially see ammonia in your tank. It will spike and then you will see nitrites start to increase. Over time, as nitrates ramp up, you will see ammonia and nitrite fall. When the levels of nitrite and ammonia reach 0ppm (parts per million), your tank has been officially cycled and you can safely add fish. 

Testing every day is recommended but you can get away with testing every two or three days if you’re stressed for time.

Step 5: Add more fish

Now that the ammonia and nitrite levels have reached zero, you can start adding more fish.

However, this needs to be done gradually, only introducing one or two fish at a time. It’s super important to make additions slowly. Adding too many fish too quickly will spike the ammonia and nitrite levels, stressing your fish. 

After each addition, wait a week and test the water again. If the ammonia and nitrite levels are still low, you’re in the clear to add more fish.

How to Cycle a fish tank without fish (recommended)

This is the most humane way to cycle your tank and for this reason, this is our recommended method to cycle an aquarium. 

Step 1: Getting the Ammonia started

In a fishless cycle, you need a source of ammonia to get the process started. But you don’t have fish. What do you do?

There are a few ways to do this. First, you can get the process started by dropping a few flakes of fish food into the tank. Pretend you’re feeding fish if you need an idea of how much food to give. You can do this every 12 hours.

Now, just wait. Over a few days, the flakes will begin to decay – releasing ammonia into your tank.

Another option is using a pure ammonia product to trigger the cycle.

Step 2: Testing for Ammonia

You’ll need a test kit to monitor the ammonia levels in your tank for a few days. Your goal amount here is at least 3ppm (parts per million).

If you’re coming up short, add more flakes and let them decay.

Test every other day, with the goal to maintain ammonia levels at 3ppm. If your ammonia level drops below 3ppm, add more flakes and let them decay. Continue this process for a week.

This 3ppm level is sufficient to trigger Nitrosomonas to grow and start consuming the ammonia, which begins the second phase of the process.

Step 3: Testing for Nitrites

Once your week of ammonia at 3ppm is finished, it’s time to test for nitrites. Use a water test kit for this process.

As soon as nitrites are detectable, the cycle has officially started. Congratulations! Keep adding ammonia as you did before to continue the cycle.

Step 4: Testing for Nitrates

After a few weeks, nitrite levels will begin to drop – which is your signal to start testing for nitrates.

When nitrates show up on your test results, the nitrogen cycle is nearly completed. The final condition you need to achieve is ammonia and nitrite levels at zero.

Continue to keep an eye on your nitrate levels. While non-toxic in small doses, if your nitrate reading is above 40, you’ll need to do a water change to bring it down.

Step 5: Adding the fish

Once ammonia and nitrite levels are zero on your test, it’s safe to start adding fish.

However, don’t go wild and add ten fish at once. You need to add fish gradually. 

Only add 1-2 fish at a time and wait a week after adding before introducing more. 

It is best practice to clean the substrate with a siphon before adding fish. You will remove any decaying food trapped in the substrate, reducing the potential ammonia burden on your tank if it is released.

How to Cycle a fish tank with plants (recommended)

This method is ideal because it transforms your aquarium into a natural ecosystem, both biologically and visually. Your future fish will also thank you for their lush environment.

Rather than using a bare tank with no fish (boring!), you can immediately add live aquarium plants like Amazon Swords and Java Ferns. Spend your time focusing on growing them with good lighting, substrate, and fertilizers. We also love this method because cycling a tank can be a slow process but watching your live plants flourish helps pass the time!

Once the plants start to show new growth, the cycle is complete. New growth signals that the plants are successfully consuming ammonia and nitrates and converting them into new leaves and roots. 

From this point, you can gradually add a few fish and feed them sparingly. Wait a week and continue monitoring the water to make sure there are no ammonia and nitrite spikes. If levels stay stable, continue to add new fish in groups of 1-2 at a time.  Use the water test kit to ensure that ammonia and nitrites are at 0 ppm and nitrates stay below 40 ppm.

What do you need to perform a fishless cycle?

Aquarium Test Kit

A good aquarium test kit is a critical piece of equipment for cycling your fish tank. It is needed to understand levels of ammonia, nitrites, and nitrates and will help you understand where you are in the cycle. It will also provide information about when it is safe to start adding fish and help diagnose any issues along the way.


You will also need ammonia with no additives. Some bottles of ammonia available for purchase contain types of soaps, perfumes, or scents. You want the ammonia to be completely unscented and does not contain and surfactants. We recommend Dr. Tims Aquatics Ammonia Chloride or Fritz Pro. It is made specifically for fishless cycling aquariums.

To figure out if your ammonia is good to use, you can do a shake test. Shake your ammonia and look at the bubbles. If the bubbles go away instantly, the ammonia is fine to use. If the ammonia is soapy looking and the bubbles last for more than a few seconds, it contains surfactants. Best to find a different bottle made for fishless cycling.


Dechlorination will keep chlorine from killing off the good bacteria but it will not make your nitrogen cycle go faster. Keeping your water dechlorinated is essential for your good bacteria to colonize in sufficient numbers.

How to Cycle a Fish Tank: Cycling Shortcuts

Now you know the basics of how to cycle a fish tank. Good news! The nitrogen cycle can be sped up. If you don’t want to wait 6-8 weeks, you can use one of the techniques below.

However, you will need access to an established tank to use most of them. So if you’re just starting out with your first aquarium, this options are likely not available to you.

Add Filter Media From An Established Tank

Filter media (like bio balls, etc.) from an established tank will have beneficial bacteria already established. No need to wait for the bacteria to grow naturally with this option, so your tank will cycle quicker.

Add Gravel From An Established Tank

Established tanks with an undergravel filter are useful for jumpstarting the nitrogen cycle. Bacteria will be attached to the gravel. 

Take about a cup of gravel and hang it in a mesh bag in your filter. This is similar to using filter media from an established tank. If this is not an option, you can also lay the old gravel over the top of the gravel in your new tank.

‘Season’ Your Filter

Attach your new tank’s filter to an established tank, and let it run alongside the existing filtration system.

The bacteria from the established tank will colonize in your new filter. Let it run for roughly one week, and then move your ‘seasoned’ filter to your new tank. Say hello to a new tank with beneficial bacteria.

Warning: Your old tank can contaminate your new tank!

Using an established tank to speed up the nitrogen cycle seems like a miracle cure. However, there is a major downside to be aware of. Your established tank could contain bacteria and parasites that will infect your new tank.  Never transfer anything from a tank that’s known to be contaminated with harmful organisms unless you want two tanks with the same problem. Only use clean and safe established tanks for this purpose and if you’re not sure, it is better to be safe than sorry.

How to solve common nitrogen cycle problems

Ammonia Poisoning

This is a serious threat to your fish. Ammonia poisoning symptoms are:

  • Lethargy 
  • Loss of appetite
  • Inflamed gills, eyes, or anus
  • Red streaks in the fins
  • Gasping for air at the surface
  • Staying at bottom of tank

If you see these symptoms, the ammonia level must be addressed quickly. Left untreated, your fish will likely die from ammonia poisoning. 

To lower the ammonia, make two changes. First, perform water changes more frequently. Secondly, change a greater portion of the water each time.

Aquarium Won’t Start Cycling

Ammonia tends to start rising around day 3. If by day 5, you’re still not measuring any ammonia and you’ve double-checked that your water test kit is functioning properly, your tank may not be cycling.

The most common reasons for this situation is that there is no source of ammonia in the tank. Or, something is removing it too quickly for the bacteria to do their magic.

Try adding more ammonia – via fish, fish food, or ammonia products – dependiing on how you’re doing your cycle. If you started with a planted tank, your plants might be too effective so try removing one or two and see if there’s a change.

Once you add more ammonia and remove plants, test again after a few days. If you’re still missing ammonia and nitrites, add more ammonia. 

Ammonia Is Not Dropping (Fishless Cycling)

There are three primary reasons your ammonia might not be dropping: 

  • pH is too low (acidic)
  • Chlorinated water
  • Too much cleaning

If the pH is under 7, ammonia will be mostly present as ammonium. Nitrifying bacteria can’t eat this form so they won’t interact in productive ways. Use a pH kit to increase the level.

Using chlorinated water for water changes will kill all the beneficial bacteria. You must add a de-chlorinator to the water before putting it in your tank during water changes.

Lastly, the beneficial bacteria that breaks down ammonia lives in the gravel, filters, and decorations. If you clean too much, you’ll remove bacteria before it is colonized and established.

My Nitrate Levels Aren’t Rising 

If your Nitrate levels are stuck at 0, you’re probably killing off the good bacteria before they establish themselves.

Similar to ammonia not dropping, always de-chlorinate your water during water changes and don’t clean your tank too vigorously. A fish tank is a balancing act – while it is good to have it clean, it also needs to be natural, not sterile.

Algae Bloom

Algae loves aquariums that are cycling. Algae loves ammonia so it is a match made in heaven.

If you’re dealing with an algae bloom, try turning off your aquarium lights. If you’re using plants in your cycling, turning off the lights 24/7 isn’t an option so restrict light hours to 10 hours a day, max.

Also, avoid over-fertilising your tank if you’re using fertilizers. CO2 injectors can also be useful.

Aquarium Cycling FAQs

Does algae mean my tank is cycled?

Algae is often a sign that the nitrogen cycle is nearing completion. Algae grows when there are enough nitrates in the tank to support algae growth. You can also test for cycle completion. At completion, there should be trace ammonia, zero nitrites, and roughly 20-40ppm nitrates.

How do I know when my aquarium has cycled?

Testing for nitrogen cycle completion is the easiest and surest way to know if your tank is done. When complete, there should be trace ammonia, zero nitrites, and roughly 20-40ppm nitrates.

Will snails help cycle a tank?

Snails will create waste and therefore, ammonia. They can be used to cycle aquariums. However, one or two snails likely won’t produce enough ammonia to get the nitrogen cycle moving because their bioload is very small. Therefore, you would need a good collection (probably 10-20) to get things moving sufficiently.

What causes ammonia spikes in aquariums?

The decomposition of organic matter—aquarium plants, fish waste, and uneaten fish food—is one-way ammonia levels rise in tanks. Chemically treated tap water can be another source. Some water treatment companies use a chemical called chloramine—chlorine bonded to ammonia—as a disinfectant. If your tap water has been treated with this chemical, you’re going to have a large influx of ammonia. 

Both of these paths introduce ammonia to a tank. If there is not sufficient beneficial bacteria to break down this ammonia, it will result in an ammonia spike, which can lethal results for tank inhabitants.

How can I increase the beneficial bacteria in my aquarium?

Assuming you have some beneficial bacteria already established, you can increase the water temperature (bacteria breeds faster in warmer conditions), increase oxygen (more oxygen equals more reproduction), add filter media from a tank that is already established, and let the filter run so your bacteria can colonize the filter media.

How do you tell if there is too much ammonia in a fish tank?

First, you can test the water for ammonia levels. In a fully cycled tank, there should be trace levels to zero ammonia present. You can also look for symptoms such as lethargy, loss of appetite, and gasping for air at the surface, which indicate ammonia poisoning or stress in your fish.

Should I vacuum gravel during the cycle?

Yes, you should vacuum the gravel. Vacuuming the gravel will help control the amount of decaying food and fish waste in the tank, which will limit ammonia spikes. Keeping an eye on your potential ammonia is an important part of learning how to cycle a fish tank.

Will dead fish help cycle tank?

Yes, a dead fish will release ammonia, which will help cycle the tank. However, be wary of the way the fish died (disease, parasite, etc.) because you could inadvertently introduce unwanted organisms into your new tank, causing more issues down the road.

How to Cycle a Fish Tank: Conclusion

The Nitrogen Cycle is an integral part of fishkeeping. If you want to keep healthy and happy fish, you need to understand fish tank cycling and how to best manage the nitrogen cycle moving forward. 

Once you’ve cycled your tank and your ammonia and nitrite levels are 0, it doesn’t mean your work is done. The nitrogen cycle is continually functioning in the background of your tank. In a balanced ecosystem, it repeats itself endlessly as the different components try to remain in balance.

For this reason, we recommend weekly water tests so you can keep an eye on the level of ammonia, nitrites, and nitrates. The last thing you want is to lose a tank full of beautiful fish because you got lazy so just test your water! It is the best way to understand what is happening in your tank.

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