Skip to Content

Aquarium Nitrogen Cycle Explained: A Beginners Guide

Aquarium Nitrogen Cycle Explained: A Beginners Guide

The aquarium nitrogen cycle is an essential part of owning an aquarium, but it can be a little challenging to understand if you’re new to the hobby. So, what exactly is the aquarium nitrogen cycle?

The aquarium nitrogen cycle converts ammonia into nitrite, then nitrate, using the beneficial bacteria inside your aquarium. Both ammonia and nitrite are very toxic to your fish, so the aquarium nitrogen cycle ensures that there are no toxins left around your fish in the aquarium.

The rest of this article will give you a beginner’s guide to understanding and performing the aquarium nitrogen cycle. Let’s get started.

What Is the Nitrogen Cycle For Aquariums?

The nitrogen cycle for aquariums involves using the beneficial bacteria in your aquarium to convert harmful ammonia into nitrite and eventually into nitrate. The waste from your fish creates ammonia, bacteria convert the ammonia to nitrite, and another type of bacteria converts the nitrite into nitrate.

The aquarium nitrogen cycle is crucial, as it must be completed before adding fish to the tank. Otherwise, you’ll have to deal with New Tank Syndrome, which is fatal for your fish. However, I’ll get more into that later in the article.

Why Is The Nitrogen Cycle Important?

Now that you know what the nitrogen cycle is, it’s important to understand why it’s vital when you’re setting up a new tank to introduce fish to.

The nitrogen cycle is important because it ensures that the water you keep your fish in is free from toxins such as ammonia or nitrite. Without performing the nitrogen cycle, you risk killing or poisoning your fish when you introduce them into the tank.

This poisoning is also known as “New Tank Syndrome,” which I’ll discuss next.

New Tank Syndrome

New Tank Syndrome is when the fish you introduce into the new tank suddenly begin to exhibit strange behaviors due to the lack of the nitrogen cycle. The nitrogen cycle ensures that there are no harmful toxins around the fish, such as ammonia and nitrite.

Some symptoms of New Tank Syndrome include:

  • Fish hiding in corners
  • Fish with clamped fins
  • Fish lying around the bottom of the tank
  • Loss of coloring in the fish

The Aquarium Nitrogen Cycle Breakdown

Let’s take a look at all of the steps that make up the Aquarium Nitrogen Cycle:

  1. You feed your fish food; any uneaten food is considered waste.
  2. Your fish consume the food and excrete waste, resulting in more waste in the tank.
  3. Plants will drop dead leaves, adding more waste to the tank.
  4. The waste in your tank accumulates.
  5. The waste begins to break down, releasing ammonia.
  6. Bacteria convert ammonia to nitrite.
  7. A different type of bacteria converts the nitrite to nitrate.
  8. Nitrates are utilized by plants to grow, and any dead leaves from the plants are considered waste.
  9. During water changes, a significant amount of nitrates is eliminated.
Aqaurium Nitrogen Cycle

Additionally, in many aquariums, and even natural ecosystems, the nitrate gets converted to nitrogen gas by denitrifying bacteria.

Nitrogen Cycle: Three Main Components

Now that you know the order of the nitrogen cycle, let’s delve a little deeper. When you hear people talking about the Aquarium Nitrogen Cycle, they’re usually referring to steps 5, 6, and 7 above, as these are the main steps in the cycle.

The three main components of the nitrogen cycle are when ammonia is in the tank when it converts to nitrite, and when the nitrite converts to nitrate.

Stage 1: Ammonia

Ammonia is highly toxic to fish, so it’s vital to ensure that the nitrogen cycle is complete before risking giving your fish ammonia poisoning. Ammonia is produced in the aquarium through fish waste, which eventually breaks down and releases ammonia.

It’s important to note that ammonia is one of the most important water quality parameters that affect fish, as even at low concentrations, it causes stress and damage to the fish in the tank.

Therefore, it’s essential to test the water regularly to watch for the accumulation of ammonia. However, there are also beneficial bacteria present during the nitrogen cycle, called nitrifying bacteria, that eat ammonia in the tank to create nitrite.

Stage 2: Nitrite

In this stage, the toxic ammonia gets eaten by beneficial bacteria called nitrifying bacteria. 

Nitrifying bacteria can be found on every surface of the tank. This bacteria eat the ammonia and then creates waste — leaving behind nitrite. However, nitrite is also toxic to fish, so this isn’t the end of the cycle.

After the bacteria create nitrite, another beneficial bacteria strand eats the nitrite, then produces nitrate.

Stage 3: Nitrate

Finally, after the two cycles of eating and wasting, the bacteria produce nitrate, which is what you want in your tank for your fish. Nitrate isn’t considered toxic to most freshwater fish. 

However, there can always be too much of a good thing. Nitrate levels in your tank should stay around 40 ppm — anything above 80 ppm can be toxic to your fish.

Two Ways To Cycle Your Tank

You can complete the aquarium nitrogen cycle on your tank in two ways — one including fish and one not including fish. Which one you choose is completely up to you, but if you’re just starting out in this hobby, I would recommend cycling your tank before buying any fish, as it might be a little easier.

In both cycling methods, you want to ensure you have various surfaces in the tank, such as plants or other decor, as beneficial bacteria need the areas to grow.

The following sections will go over how to cycle your tank with and without fish.

Nitrogen Cycle Your Tank With Fish

If you choose to nitrogen cycle your tank with fish, you’ll have to take some precautions. For example, introducing the wrong fish to your tank too soon can result in them dying, so you have to ensure the first fish you introduce are hardy, which I’ll get into more detail about.

Introduce A Small Number Of Hardy Fish

During the cycling process, it’s crucial that, if you’re going to add fish early on, you only add hardy fish. This type of fish will be able to survive the high-toxin levels long enough for the beneficial bacteria to start doing their job, breaking down ammonia and nitrite into nitrate.

Some examples of hardy fish include:

  • Guppies
  • Minnows
  • Danios
  • Barbs

Additionally, don’t add too many hardy fish in the tank, and the fish you do add should be somewhat inexpensive, as they’re still at risk of poisoning. Adding one or two fish at a time is ideal.

Feed Your Fish Sparingly

When your fish are in the tank, you want to ensure you feed them sparingly. Feeding them too much will produce an overabundance of ammonia, which isn’t ideal when trying to cycle the tank.

Sparingly feeding the hardy fish will ensure the beneficial bacteria in the tank can consume all of the ammonia, so it’s less toxic to the fish.

Change The Water

During the nitrogen cycle, it’s important to change the water frequently. Many people skip this step because they believe changing the water is removing the beneficial bacteria from the tank, but this isn’t true — beneficial bacteria only live on surfaces, not in the water. Therefore, changing the water doesn’t hurt the bacteria.

Changing the water can help control the amount of ammonia in the tank during the beginning stages of the nitrogen cycle, and this is important when there are fish already in the tank. I recommend changing the water every few days.

Test Toxin Levels

During the cycling process, and especially with the addition of fish, you’ll want to regularly test the toxin levels in the tank for ammonia and nitrite. While there are signs you can look for in the aquarium to understand where your tank is in the nitrogen cycle, testing the toxin levels will give you a more accurate picture.

You’ll need to purchase a test kit to test the toxin levels in the tank. You can find these test kits in any pet store, but for ammonia, I recommend the DIP & GO Ammonia Test Kit from This test kit quickly and accurately tests the ammonia levels in your tank, giving you results within 10 seconds.

For testing nitrite, I recommend the API Nitrite Test Kit from This kit is easy to use, and it’s made to use as a weekly monitoring of nitrite levels in the tank.

Gradually Add More Fish

Once the toxin levels are decreasing, and the cycle is working as it should, you can gradually add more fish. Again, don’t add too many at a time, as this can mess up the system and cause toxins to rise again.

It’s ideal to only add one or two fish at a time for a while and continue to test the water for toxins.

Nitrogen Cycle Your Tank Without Fish

The other way you can cycle your tank is without fish, which is a bit easier because you don’t have to worry about the health of the fish during the process.

So, let’s go over the steps you should take when doing a nitrogen cycle on your tank without fish.

Start The Ammonia Process

Beginning the ammonia process without fish is different than with fish, as you don’t have the fish food and waste to rely on.

Therefore, you’ll have to manually introduce ammonia to the tank. After you set up your tank and add the various plants and decor, add five drops of pure ammonia per ten gallons of water daily.

Doing this will cause the ammonia levels in the tank to rise drastically.

Test Ammonia Levels

As you’re adding ammonia drops to the tank daily, begin testing the water for ammonia. You can do this by using the testing kits I mentioned in the previous section.

The ammonia levels should rise to 5 ppm and higher. Once you see this, start testing for nitrites.

Test For Nitrites

Again, you can test for nitrites using the kit mentioned above. Once you start seeing nitrite levels rise, reduce the amount of pure ammonia you’re adding to the tank. After a while, the nitrites and ammonia will be on similar toxin levels.

Test For Nitrates

When you see similar levels of ammonia and nitrites in the tank, it’s time to start testing for beneficial nitrates. You can also do this using a testing kit — I recommend the API Nitrate Test Kit from Like the nitrite testing kit, this kit is straightforward to use and can be used weekly for accurate testing.

Once you see a rise in nitrates and zero levels of ammonia and nitrites, your tank is finished cycling.

Gradually Introduce Fish

Finally, with adequate levels of nitrates in your tank, you can gradually start introducing fish to the aquarium. However, although there are no toxins in the tank, it’s still ideal to introduce fish one or two at a time and continue testing the water. 

With the addition of fish waste and fish food to the tank, you can expect ammonia levels to rise again, which is why gradually introducing fish is best.

How Do You Know When The Nitrogen Cycle Is Complete?

You know when the nitrogen cycle is complete when ammonia and nitrite are testing at zero ppm, and nitrate levels are around 40 ppm. If there are any leftover ammonia or nitrite levels in the tank, the cycle hasn’t been completed yet.

Nitrogen Cycling Common Problems & Treatment

Unfortunately, during the nitrogen cycle, you can run into various problems. Luckily, however, these problems typically come with treatment plans. Some of these problems may include the following:

  • Ammonia poisoning in your fish
  • The ammonia levels not dropping
  • The aquarium is not cycling
  • An abundance of algae during the cycle
  • Nitrate levels not rising

Let’s review the causes, signs, and treatments of each problem.

Fish-In Cycling: Ammonia Poisoning

One of the worst possible outcomes of choosing to do a nitrogen cycle with your fish in the tank is giving your fish ammonia poisoning. 

Ammonia is tricky to work with, as too much poses a severe risk to your fish. There can be many reasons why your fish are dealing with ammonia poisoning, so let’s look at the causes, signs, and treatment options.

Causes of Ammonia Poisoning

Your fish suffering from ammonia poisoning is due to high ammonia levels in the tank. The high levels of ammonia can be due to the following reasons:

  • You have too many fish in the tank.
  • You’ve added pure ammonia to the tank, along with your fish.
  • You haven’t done any (or enough) water changes.
  • The nitrite levels are too low.

Signs of Ammonia Poisoning

Some signs of ammonia poisoning in your fish include:

  • Red, bleeding gills or burns
  • Laying on the bottom of the tank
  • Darkening body color
  • Increased infections
  • Death

If you believe your fish are suffering from ammonia poisoning, it’s essential that you work to fix the problem as soon as possible. 

Treatment for Ammonia Poisoning

Unfortunately, if the ammonia poisoning is severe, your fish most likely won’t survive. However, if you catch the signs of ammonia poisoning in time, you can save your fish.

It’s essential that, at the first sign of ammonia poisoning, you change 50% of the water in your tank. This will lower the levels of ammonia, which will hopefully allow your hardy fish to survive.

Additionally, it’s best to restrict feeding your fish for a while to ensure there’s less waste converting to ammonia in the tank.

If your fish have ammonia burns, you might have to quarantine them in another tank and treat them with antibiotics.

Fishless Cycling: Ammonia Not Dropping

While doing a nitrogen cycle without any fish involved relieves you of the stress of possibly poisoning your fish, you still have other issues you can run into. One of these issues is ammonia levels not dropping, which prevents your aquarium from completing the entire nitrogen cycle.

Let’s take a look at the causes and treatment of ammonia levels not dropping.

Causes of Ammonia Not Dropping

There are three main causes of ammonia not dropping:

  • pH too low: When the water’s pH levels are below 7, nitrifying bacteria typically won’t feed off the ammonia, which leaves the ammonia levels high in the tank.
  • Chlorinated water: If you use too much chlorinated water, it will kill the beneficial, nitrifying bacteria in the tank — leaving nothing to feed off the ammonia.
  • Too much cleaning: Cleaning the tank too often will also eliminate the beneficial bacteria in the tank, leaving behind ammonia.

Treatment For Ammonia Not Dropping

The best way to treat ammonia not dropping in your tank is to first discover the primary cause. If the pH levels are too low, use a pH kit to raise the level.

Additionally, avoid using chlorinated water, and refrain from cleaning the tank so often.

My Aquarium Won’t Start Cycling

Waiting for your aquarium to start cycling but nothing happening is another common problem you might encounter, especially if you’re new to the hobby. There are a few things that can cause this.

Causes of Aquarium Not Cycling

If your aquarium isn’t cycling as it should be, you might not have a significant enough source of ammonia in the tank. This is especially common if you’re cycling your tank without the use of fish, as there’s no fish waste.

However, there could also be an abundance of beneficial bacteria that are eating ammonia too fast.

Treatment of Aquarium Not Cycling

Whatever the cause may be, adding ammonia to the tank will usually get it to begin cycling. If you’re cycling the tank without fish, add more drops of pure ammonia to the tank. If you’re cycling with fish, ensure you feed them enough.

Additionally, you can add more surface areas to the tank to give the beneficial bacteria somewhere to grow, thus, increasing cycling.

Fishless Cycling: Algae Bloom During Aquarium Cycling

Seeing algae bloom during your nitrogen cycle can be worrisome, but it’s actually a normal occurrence, at least at the beginning of the cycle. Therefore, unless the algae bloom out of control, it’s not something you really need to worry about. Nonetheless, let’s go over some causes, signs, and treatment options for algae bloom during the nitrogen cycle.

Causes of Algae Bloom During Cycling

Causes of algae bloom during cycling include:

  • An abundance of ammonia
  • Not enough beneficial bacteria in the tank
  • Bad water circulation
  • Poor tank maintenance

Signs of Algae Bloom During Cycling

You’ll notice algae bloom beginning when the water starts to turn green. Unfortunately, algae bloom suspends in water rather than sitting on surfaces, so it’s not something you can easily clean. Therefore, it’s challenging to remove it completely from your tank.

Treatment of Algae Bloom During Cycling

Most of the time, you won’t need to treat algae bloom during cycling. Algae is a good sign that the nitrogen cycle is nearing completion, as the formation of algae is a sign that nitrates are in the water.

However, if you feel like you have excess algae in your tank, performing regular water changes will help reduce it.

Fishless Cycling: Nitrate Levels Aren’t Rising Enough

Lastly, you might run into the problem of nitrate levels not rising enough. This is a common problem during fishless cycling, and without rising nitrates, you can’t add fish to your aquarium. Let’s go over the causes and treatment of nitrate levels not rising enough.

Causes of Nitrate Level Not Rising Enough

If your nitrate levels aren’t rising enough, it’s probably because you’re killing off the beneficial bacteria in the tank before they can convert the nitrites into nitrates. This is why consistently testing your water and watching the nitrogen cycle progress is so essential.

Treatment of Nitrate Level Not Rising Enough

The most important thing you can do when you realize your nitrate levels aren’t rising enough is to ensure you’re not using chlorinated water. Chlorinated water will kill off all of the beneficial bacteria in your tank, thus, ending the nitrogen cycle prematurely.

Additionally, ensure you don’t over-clean your tank or change the water too often.

Aquarium Nitrogen Cycle FAQs

Are Water Changes Necessary During Cycling?

Water changes aren’t necessary during cycling, but it is encouraged. Because the beneficial, nitrifying bacteria live on surfaces, changing the water doesn’t affect their development. Therefore, changing the water is encouraged to keep the tank clean.

How Long Does Cycling An Aquarium Take?

Cycling an aquarium takes anywhere from four to eight weeks. The nitrogen cycle has to go through three stages: the development of ammonia, converting ammonia to nitrite, and converting nitrite to nitrate. The cycle is completed when there are zero levels of ammonia and nitrite in the tank.

Does Brown Algae Mean My Tank Is Cycled?

Brown algae does not mean your tank is cycled, but it’s a good indicator that it’s close to being finished. The presence of brown algae is a sign that there are enough nitrates in the tank to support the algae. Therefore, you can expect your tank’s nitrogen cycle to finish soon.

What Happens If You Put Fish In An Uncycled Tank?

If you put a fish in an uncycled tank, you risk giving your fish “New Tank Syndrome” or ammonia poisoning. Adding fish to an uncycled tank will cause an ammonia spike, as fish food and waste convert into ammonia. Therefore, with more ammonia and no nitrates, your fish will suffer.

Does Water Conditioner Remove Ammonia?

Water conditioner does remove ammonia. Along with neutralizing ammonia, water conditioner can remove chlorine and other harmful chemicals from the water while helping keep nitrates at a manageable level.

Wrapping Up

The aquarium nitrogen cycle is an essential part of caring for fish. Therefore, understanding the process before diving into the hobby is critical.

The nitrogen cycle consists of three stages: adding ammonia into the tank (whether in the form of ammonia drops or through fish waste), nitrifying bacteria converting the ammonia into nitrites, and another strand of bacteria converting nitrites into nitrates.

While you can complete the nitrogen cycle both with fish and without, it’s best to do it without. Otherwise, you risk giving your fish ammonia poisoning.

I’m Elle, the founder of FishHQ. I created this website to share knowledge, tips, and inspiration for beginner hobbyists to help them create a healthy, happy, and vibrant environment for their fish to thrive. Read more...