How to Cycle Your Aquarium? What is Nitrogen Cycle?

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Any beginner’s guide to fish keeping will cover the cycling of your aquarium, which is the process of establishing a beneficial bacterial colony inside your tank before adding any fish to it.

In this guide to aquarium cycling I will talk about the Nitrogen Cycle and the various methods you can use to achieve a healthy and stable environment for your fish.

I’ll also discuss some of the ways you can speed up the aquarium cycle, some of the ways you shouldn’t speed up your cycle and some of the problems that may arise during the Nitrogen Cycle and their fixes.

Before we get into the details on how to cycle your aquarium, let’s see what it means to cycle your aquarium and why it’s a must for every tank.

What Does the Nitrogen Cycle Mean?

If you were to take your newly bought fish tank, fill it with water and add fish in it, these fish will probably end up dead.

Why? Because of the accumulation of ammonia in the tank from waste produced by fish, which is lethal to them.

Performing the nitrogen cycle in the tank before adding your fish prevents your fish from dying.

By establishing and creating a beneficial bacterial colony, ammonia in your tank gets transformed into nitrite, then nitrates.

As we already established, ammonia is toxic for your fish, and so is nitrite, however, normal levels of nitrates in your tank are non-toxic.

To keep nitrates at normal levels in your tank, you’ll have to clean your tank, i.e., perform regular water changes. Otherwise, your fish can lose their appetite and color.

Also, high levels of nitrates can favor algae blooms, which can also harm your fish.

Once there’s a colony of beneficial bacteria in your tank and filter media, the waste of your fish is neutralized by bacteria and harmful ammonia is converted into non-toxic nitrates.

What’s the Duration of the Nitrogen Cycle?

It can take a while for bacteria to colonize your tank and sometimes it can be confusing to tell if the cycle is complete or not.

Before you start a fish tank cycle process, I advise you to get an aquarium test kit and arm yourself with some patience.

The nitrogen cycle can take two to six weeks, so getting a test kit will help you know at which stage you are in and how you’re progressing with the tank cycle each week.

You’ll have to test the water in your tank for ammonia, nitrites and nitrates every other day.

In the incipient phases of the nitrogen cycle, ammonia levels will peak, then as soon as nitrite levels start to take hold, ammonia levels will drop. Nitrite levels will start to fall as soon as nitrate starts to form.

You can safely add your fish to the tank as soon as you no longer detect nitrites in your tank.

The actual duration of the nitrogen cycle can be influenced by the following factors:

  • Availability of beneficial bacteria in the atmosphere that can colonize the filter initially;
  • Concentration of ammonia that bacteria can feed on;
  • Frequency and amount of water changes for the duration of the cycle;
  • Amount of decaying plants, food, or dead fish in the tank;
  • Source of waste that can feed the developing bacteria;
  • Presence of pathogens or chemicals in the tank.

Before I discuss the nitty-gritty of how to cycle your aquarium, I’ll walk you through the three stages of the nitrogen cycle.

Stages of the Nitrogen Cycle

As I already mentioned, the aquarium cycle can be divided into three stages based on the chemical reactions that occur within the tank.

These stages are:

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


Ammonia is released into the tank as a result of fish waste or uneaten food and keeps on building up until ammonia-eating bacteria form.

When this happens, you’ll notice water in the tank getting cloudy.

Test the water for ammonia and if levels are decreasing, your tank is entering into stage two of the nitrogen cycle.


As ammonia is oxidized by a bacterium called Nitrosomonas, it turns it into nitrite. As the levels of ammonia decrease, the levels of nitrites start to climb.

Now, just because you got rid of ammonia, which is toxic to your fish, it doesn’t mean you can start adding fish to your tank.

Nitrites are still toxic, so you’ll have to wait for the third stage of the nitrogen cycle to kick in, the nitrate cycle.

With high levels of nitrites, a colony of bacteria that eat nitrites forms and disposes of it. This is when nitrite levels plummet and nitrates take over.

You’ll notice nitrite levels dropping by the end of the first week, beginning of the second week of the nitrogen cycle.


This is the third and last stage of the aquarium cycle. At this point bacteria called Nitrobacter develop that convert nitrites to nitrates.

If your aquarium water testing returns results of 0 parts per million ammonia and nitrites, it means your aquarium cycle is complete.

But just because the cycle is complete, it doesn’t mean you can stop monitoring the water in your aquarium.

I said earlier that normal levels of nitrates aren’t harmful to fish, but once levels get out of hand, problems do arise.

Continue to monitor nitrate levels and keep them under 20 parts per million by performing regular water changes of 20 to 50% weekly or monthly depending on the nitrate readings in your tank.

These water changes are useful to remove waste, decaying materials, and replenish your tank with healthy minerals and nutrients.

In freshwater tanks, plants may also help keep nitrate levels under check, while deep sand beds and live rock can do the same in saltwater tanks by allowing denitrifying bacteria to form.

Now that you have a clear understanding of the various stages involved in the aquarium cycle, I can go on to discuss the methods you can use to cycle your aquarium.

How to Cycle Your Fish Tank?

There are two aquarium cycling methods that I’ll be discussing:

  1. Cycling the aquarium without fish;
  2. Cycling the aquarium with fish.

The first method is, in my opinion, the right method to do it from an ethical standpoint. Why? Because with the other method, you’ll probably kill most if not all your fish.

So, out of a preference for the first method, I’ll discuss it first:

Method #1: Cycling Your Aquarium Without Fish

A fishless tank cycle is the most humane way to establish a healthy environment for your fish. It guarantees that no fish are harmed in the process.

This is a beginner-friendly 5-step process that will get you the desired results even without having fish in your tank.

So, let’s get to it:

Step 1: Adding Ammonia to the Tank

I mentioned that the first stage of the nitrogen cycle is creating an environment rich in ammonia.

If there’d be fish in your tank, they would naturally create ammonia through the waste they expel. But because you don’t have fish waste lying around in your drawer, you’ll need to get creative.

To overcome this hurdle, you can start by adding flakes of fish food to the tank just as you would when feeding your fish. Do this regularly every 12 hours.

With time — and this is where patience comes in! — food will decay and it will release ammonia into the tank.

Step 2: Monitor Ammonia Levels

Get your aquarium testing kit out and test ammonia levels in your tank every other day. If you’ve reached levels of 3 parts per million, you’re doing it right.

If levels are below this, keep adding fish food to the tank. Once you reach 3 ppm try and maintain this level, so ammonia-eating bacteria (Nitrosomonas) can develop.

These bacteria will start consuming ammonia, so you should continue monitoring the water and keep adding flakes if no notice levels dropping below 3 ppm.

After a week of offering sustenance for these bacteria, you can head on to the next step.

Step 3: Monitoring Nitrite Levels

After week one ammonia levels peak and they’ll start to plummet once nitrites take over, and so it’s time to test for nitrites in your tank.

If your measurements detect nitrites, you’ll know you’ve graduated to stage two of the nitrogen cycle.

Continue to add ammonia and monitor nitrite levels.

Step 4: Monitoring Nitrate Levels

Once you enter into stage 3 of the nitrogen cycle, nitrite levels will drop, and you can start testing for nitrates.

Once you detect nitrates, you’ll know the cycle is nearing its completion.

The cycle is complete when you detect nitrates, but you no longer detect ammonia or nitrites.

Now, all you have to do is monitor nitrate levels and keep levels below 40 by performing regular water changes.

Step 5: Adding Your Fish

Now that your ammonia and nitrites reading is zero, you’re out of the woods and start gradually adding fish to your tank.

Still, before you do that, I strongly recommend siphoning the tank substrate to remove any leftover food, which may be decaying.

If the substrate is disturbed by fish scavenging in the substrate, it can release ammonia into the tank and possibly perturb the chemical balance in the tank.

I also advise against adding all your fish into the tank at once. Add a few at a time, then a few more in 1-2 weeks.

This is how you cycle your fish tank if there aren’t any fish in it. Let’s see the other method, the one that is done with a tank that has fish in it.

Method #2: Cycling Your Aquarium with Fish

You could say this is the “old school” method to cycle your tank.

I don’t recommend doing this, because you’ll basically sentence your fish to death or in the very least stress them out and expose them to diseases.

However, if you happen to buy the tank and the fish on the same day, you’re pretty much stuck with this option.

So, here’s how to do it:

Step 1: Add a Small Number of Hardy Fish

Some types of fish are better at handling high ammonia and nitrite levels, these are the so-called hardy fish.

If you’re a beginner aquarist, chances are the first fish you buy are beginner-friendly hardy fish, or at least a good pet store will recommend these over delicate fish.

Fish will produce waste that will get the ammonia started and if they’re hardy, they’ll hopefully also survive the ammonia and nitrite levels until beneficial bacteria can colonize the tank.

To keep things as balanced as possible, don’t add more than 1-2 fish per 10 gallons of water.

Adding too many fish during the cycling process is a bad idea, and here’s why:

  • More fish means more waste that’s being produced, and a higher chance of stress, diseases and die-off;
  • A high number of fish added for tank cycling purposes increases the risks of water problems that can arise during the nitrogen cycle;
  • Cycling with a high number of fish can lead to foul smells coming from the tank.

Therefore, I recommend adding only a few fish, so you can more easily manage the chemical changes that occur in the stages of the aquarium cycle.

The following fish species are hardy fish that can be a good option for cycling your tank: Minnows, Guppies, Zebra Danios, Banded Gouramis, White Clouds, X-ray Tetras, Tiger or Cherry Barbs, Pseudotropheus Zebra, and Pupfish.

Step 2: Start Feeding Your Fish

To help ammonia get started, you need to feed your fish, so they can produce waste.

However, you shouldn’t overfeed them, in fact you should only feed every other day with moderate amounts of food.

If you feed them too frequently, they’ll produce too much waste, which will increase the toxicity level of the tank, before bacteria can form.

Having leftover food in the tank is also bad, because it will start decaying, which will once again release ammonia, making your tank even more toxic.

Step 3: Clean Your Tank

By cleaning your tank, I mean performing regular water changes keep toxic ammonia and nitrate levels under check.

For this process, you shouldn’t use chlorinated water (if you’re using tap water, de-chlorinate it first) because chlorine will kill off beneficial bacteria.

When performing water changes, remove no more than 10-25% of water every 2-3 days to prevent removing too much ammonia and nitrites, which beneficial bacteria will need to feed on.

Both chlorine and very low levels of ammonia and nitrites can undermine the aquarium cycling process.

Step 4: Monitoring Toxin Levels

You can test ammonia, nitrite and nitrate levels every day or every other day. It’s important to keep checking toxin levels for two reasons:

  • First, you’ll know in which stage of the cycling process you are;
  • Second, you can tweak levels if readings are off.

When levels of ammonia and nitrite drop to zero, you’ll know that the cycling process is completed.

Overall, the signs that the nitrogen cycle has reached its completion include:

  • There have been no signs of ammonia stress in your fish for at least two consecutive weeks of weekly water changes;
  • When testing the water for nitrite and ammonia, readings remain at 0 for both for at least 8 consecutive days;
  • Nitrates test indicate that nitrates are rising in the water.

If everything seems in order, you can populate your tank with more fish.

Step 5: Adding the Rest of the Fish

With toxins eliminated from the tank, you’re good to go on adding more fish to the tank. Like I mentioned when I discussed fishless cycling, you shouldn’t add a whole lot of fish at the same time.

What you should do instead is to add them gradually, one or two at a time. Wait a few weeks, while monitoring ammonia and nitrite levels, then add some more fish if levels are low, and so on.

Adding all fish at once will throw off the balance of the aquarium and stress your fish to the point where they can become sick.

This is how you cycle your tank when you have fish in it.

But what if you really don’t want to go through the whole ordeal and wait weeks on end until your tank is cycled?

Isn’t there a faster way to cycle your fish tank?

If you’re eager to get things started fast, there are ways to speed up the aquarium cycle.

How to Speed Up the Aquarium Cycle?

The following ways to jump start biological colonies in a fish tank require access to a stable, established tank.

With some caveats, these methods will shorten the aquarium cycle:

“Seeding” Your Tank

There are various ways to “seed” your aquarium, i.e., jumpstart a biological colony:

  • Adding filter media from an established tank;
  • Adding gravel from an established tank.

When you add filter media from an established tank, you’ll transfer nitrifying bacteria with the filter media, and thus, you won’t have to wait for these bacteria to grow naturally. This will speed up the cycle.

Likewise, when adding gravel from an established tank, you’re transferring biological colonies.

You’ll only need a cup of gravel, which you can lay down on top of the gravel in your tank or hang it in a mesh bag in your filter.

“Seasoning” Your Filter

This method also relies on “borrowing” bacterial colonies from an established tank. Add your new filter to an established tank and let it run along the filter in the stabilized tank.

After a week of running the new filter in the mature tank, you can remove it an add it to your new tank.

Live Plants

Alternatively, you can use live plants sourced from a mature tank and add them to your new tank. This will help speed up the process.

Live plants like Vallisneria and Hygrophila from a mature tank will not only expedite the nitrogen cycle, they will also help moderate ammonia levels in your tank through a process called protein synthesis.

What About Using “Boosting” Chemicals?

If you go to a fish store, you’ll see various nitrogen cycle enhancement chemicals designed to speed up the cycle.

Should you use those instead of the methods I mentioned above? Or should you just wait things out and let things run their natural course?

First, let’s see some of the chemicals you’ll come across:

  • Chemicals to stop unwanted ammonia increases;
  • Bacteria boosters.

If there’s too much ammonia in your tank and you notice signs of ammonia poisoning in your fish when doing a cycle with fish in your tank, then it means you’re either overfeeding your fish, your tank is overpopulated, or you’re under-filtering the tank.

Throwing a chemical to lower ammonia levels, won’t fix the source of the problem, only the symptom, so I don’t really recommend using it.

I’m also not in favor of using bacteria boosters, because these lead to a less stable tank than one that’s cycled slowly and naturally.

On the long run, bacteria booster chemicals can take months to stabilize after the last treatment, so if you don’t want to experience water issues because of bacterial disbalances, you should forgo the use of these chemicals.

The Caveats to Speeding Up the Nitrogen Cycle

Before you use any of these methods to speed up the fish tank cycle, know that there are risks associated with them, beyond the risks I discussed when I tried to dissuade you from using booster chemicals.

Transferring gravel or filter media from an established tank, could also transfer some unwanted pathogens that will cause issues.

Therefore, make sure you don’t transfer anything from a tank that’s known to have pathogens or diseases.

Another issue is transferring gravel or filter media from a tank with alkaline water to a tank with acidic water. This will kill off the beneficial bacteria you’re hoping to transfer.

To prevent this from happening, make sure that the water parameters in the two tanks are similar.

Whether you do a fishless cycle, a cycle with fish, or use any of the cycle speed-up methods I described above (maturing a filter, adding gravel or filter media from a mature tank), one thing is certain — you’ll need time and patience to complete the cycle.

Whichever method you use, you can’t escape the process of testing the water in your tank for ammonia, nitrites and nitrates, and performing regular water changes.

If you’re serious about fishkeeping, this is something you simply must do in order to ensure a healthy environment for your fish.

So far, I’ve discussed the normal sequence of things in a nitrogen cycle and the various ways you can approach or speed up the cycle.

But things not always progress how they should be, so to prepare you for when things go south, I’ll go over the most common problems that can arise in the cycle and how you can solve or mitigate them.

Common Aquarium Cycle Issues & Fixes

There are at least 5 types of cycle issues that can arise during the nitrogen cycle process, and these are:

  1. Fish poisoning caused by ammonia;
  2. Cycling won’t start because of low ammonia levels or no ammonia;
  3. High ammonia levels that don’t drop;
  4. Algae blooms;
  5. Nitrate levels won’t rise.

Let’s see what could be causing each of these issues and how can you fix them.

1.  Fish Poisoning Caused by Toxic Levels of Ammonia

This can happen if you’re cycling an aquarium with fish in it and this is why humane aquarists advise against this aquarium cycling method.

As we’ve established before, ammonia is toxic to your fish. If you’re doing a cycle with fish in it, look for early symptoms of ammonia poisoning:

  • Loss of appetite;
  • Lack of movement or lethargy;
  • Inflammation of gills, anus, or eyes;
  • Sinking to the bottom of the tank;
  • Red streaks in fins;
  • Coming up to the surface for air.

Knowing about these symptoms can decrease the likelihood of something bad happening to your fish.

If you notice these symptoms in your fish, it’s time to spring into action and perform water changes more frequently and in higher volumes to dilute the ammonia in you tank.

If you don’t take measures to lower ammonia levels in your tank, your fish can and will die.

2.  No Signs of Cycling

While ammonia levels can get out of hand, there’s also the possibility for it not to rise. Generally, by day 3 you should see a rise in ammonia levels.

If by day 5 there’s still no nudge in ammonia levels, it’s possible that something may be doing away with ammonia faster than bacteria can get to it.

Just to be on the safe side, make sure your testing kit is working properly, it’s not unheard of that the kit could be throwing out bad readings.

If this in not the case, try adding more ammonia to the tank and if you have a planted tank remove some live plants to see if there’s a change.

Wait a few days, test again, and if there’s still no rise in ammonia or nitrites, continue adding more ammonia until it starts to appear.

3.  No Drop in Ammonia Levels

Although ammonia levels are more likely to reach high levels in a tank with fish, it can also happen in a fishless tank.

The reasons could be:

  • excessive cleaning of the tank;
  • use of chlorinated water;
  • Low water pH.

If you’re cleaning decorations, filters, and the gravel too often, you’re removing and killing off beneficial bacteria that reside in your tank.

Thus, there’s no bacteria to convert ammonia and levels remain high.

The use of chlorinated tap water without any de-chlorination is another mistake that could explain the high levels of ammonia in your tank.

Again, you’re killing off ammonia-eating bacteria, so there’s no way levels can drop.

A pH level below 7 is another reason for high readings of ammonia. When pH is low, there’s ammonium in your tank, which bacteria won’t feed on.

The solution is to increase pH levels, so nitrifying bacteria can feed off of ammonia.

4.  Algae Blooms

Algae blooms can affect your tank even in the nitrogen cycle, so keep an eye on your tank to avoid algae blooms.

You can keep algae blooms under check if you observe the following rules:

  • Don’t keep your fish tank in direct sunlight;
  • Remove dead vegetation and uneaten food because these can feed algae;
  • Don’t leave aquarium lights on for more than 10 hours a day or turn them off completely of you don’t have plants in your aquarium;
  • Carry out regular water changes;
  • Avoid over-fertilizing the tank.

These methods can prevent algae blooms during the nitrogen cycle and beyond.

5.  Low Nitrate Levels

Nitrate levels could not be rising for the same reasons ammonia levels won’t drop in your tank, and that is because you’re using chlorinated water, which kills off beneficial bacteria, or because you’re doing too much cleaning in the tank.

Final Thoughts

So, you’ve completed the nitrogen cycle, avoided some of the problems that could arise, added the fish, and everything seems to be in order.

You can now sit back and relax, right?


I hate to burst your bubble, but no, you can’t just sit back and relax, because the cycle keeps on repeating itself in your tank as part of the process of creating a balanced ecosystem.

So, what does this mean for you?

Although you won’t have to keep such a close eye on the chemistry of your tank as you did during the nitrogen cycle, you’ll still need to monitor ammonia levels from time to time, and you’ll need to perform regular water changes.

Doing these will prevent subsequent issues like disturbance to the bacterial colony in the tank, algae blooms, and ammonia spikes.

I hope my guide to aquarium cycling has covered all that you need to know about the nitrogen cycle and you are now ready to get started on your new tank.


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Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.

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