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How To Cycle A Fish Tank: Navigating The Nitrogen Cycle


Staff member
Jan 19, 2024
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A lot of beginning aquarists are intimidated by the nitrogen cycle and how to cycle a tank. People ask tons of questions about this, learn all the information, and are still scared of the process.

Why is this?

The answer is simple. Correctly managing the nitrogen cycle in your aquarium isn’t an option if you want to own fish, it’s a necessity.

Failing to do this will lead to serious (often fatal) health consequences for any life in your tank. This could be caused by letting ammonia get out of control with your current tank, or mismanaging the process when cycling your aquarium for the first time.

So yeah, it’s not surprising that new aquarists get a little freaked out when it comes to this topic.

Luckily, cycling your tank and understanding the nitrogen cycle aren’t things that you need to be afraid of. In fact, once you understand the process from a scientific standpoint everything starts to fall into place.

That’s why we put together this info guide. In it, you’ll learn everything you need to know about the nitrogen cycle and how to properly cycle a fish tank on your own.

By the time you’re done, this whole process will seem like a piece of cake!

What Is The Nitrogen Cycle?

The nitrogen cycle is the process in which beneficial bacteria initiate a positive conversion process that allows toxic ammonia to be removed from the aquarium.

An illustration of the nitrogen cycle

What the heck does that mean?

Ammonia is toxic, which means it has to be managed if you want the life in your tank to thrive. Fortunately, that’s where the good bacteria come in. These bacteria colonies kick off the process by enabling this toxic ammonia to turn into nitrite.

But there’s a little problem, nitrite is toxic too.

Thankfully the process doesn’t stop there. This nitrite then converts into nitrate, which isn’t toxic. This awesome three-step process is absolutely essential to the well-being of your tank.

Too much ammonia in your tank can kill its inhabitants and excessive nitrate levels can result in algae bloom and a drop in tank water quality (which can cause problems as well).

Now that you have a basics understanding of what the nitrogen cycle actually is, let’s dig a little deeper. While you might not want to get super scientific we think it’s helpful to know as much as you can about this.

The most experienced and successful aquarists we know all have an impressive understanding of the nitrogen cycle and use it regularly to help their tanks thrive.

The Different Stages

In the previous section, we briefly covered the individual stages of the nitrogen cycle. Now it’s time to explore each one in a bit more detail.

The Ammonia Stage

As we touched on earlier, the presence of ammonia in your tank is something that has to be dealt with. This is caused by an accumulation of organic matter in your tank like waste or food that hasn’t been eaten.

Without beneficial bacteria, the levels of ammonia in your tank will build up unchecked and cause serious problems with the fish in your tank. While some fish are more resistant to ammonia than others, here’s what they’ll feel:

  • Burning in their gills
  • Elevated stress levels due to discomfort
  • Organ and brain damage (if the water has high ammonia levels)
  • Possible death

Not fun.

That’s why the beneficial bacteria is so important to the nitrogen cycle. It will allow the ammonia to convert into nitrite in the second stage of the process.

The Nitrite Stage

As your ammonia starts to convert to nitrite you’ll naturally see the amount of nitrite in your tank rise. This is something that you can (and should) monitor consistently.

The way this happens is through a metabolic process called nitritation. Yes, that’s a real word.

This metabolic process is initiated by a certain kind of bacteria called Nitrosomonas. Not only does this bacteria help oxidize ammonia into nitrite, but they also keep carbon dioxide fixation in check as well.

But nitrite is toxic as well, so something has to be done with it. This is where the beauty of the nitrogen cycle comes through. Just like we did with the ammonia, the levels of nitrite have to rise and accumulate to a point where beneficial bacteria come to the rescue again.

You’ll know when things are moving on to the third and final stage when your tests show a consistent increase in nitrite.

The Nitrate Stage

We’ve finally arrived at the last stage of the nitrogen cycle. By now the nitrite in your tank has reached the tipping point and good bacteria is rushing to the rescue.

The name for the bacteria that handle this process is Nitrobacter. These bacteria oxidize nitrite into nitrate by feeding off the energy from nitrite ions.

You’ll know that this process is complete when the ammonia levels are at 0ppm in your tank.

Author Note: It’s important to note that too much nitrate can cause problems in your aquarium as well, there just has to be a LOT of it. You can reduce this by being consistent with partial water changes, using proper gear like a protein skimmer, and adding plants to your tank.

How Long Does The Process Last?

For the most part, the nitrogen cycle should last somewhere between two and six weeks. However, there are a number of things that can impact this time range:

  • How much ammonia you’re starting with. More ammonia = more time.
  • If you use anything like bio-media or aquarium additives. These can help speed up the process.
  • The amount of plant life and live rocks you have in your tank when the nitrogen cycle occurs. You would be shocked at how much help these can provide.

While this is happening it’s your job to perform frequent water tests using a testing kit to get an idea how far along you are in each stage and make sure that everything is going smoothly.

Fishless Cycling

Now that you know what’s actually happening to your water during the process, it’s time to show you how to cycle a fish tank yourself.

The method we’ll share is what’s called “fishless cycling.” The reason for the name is quite simple, you don’t perform this with any fish in your tank.

As you know by now, in order for the nitrogen cycle to progress from stage to stage there has to be a spike in ammonia and nitrite. These are both toxic which will make your fish very uncomfortable, stressed, sick, and even result in death (this is very common).

Author Note: This is why we only recommend that you spend your time learning how to cycle a tank without fish in it. Causing pain or killing other living things is just not how we roll.

Step 1: Introducing Ammonia To Your Tank

Without any critters in your new aquarium to produce fish waste, you need a workaround in order to actually introduce a source of ammonia into your tank. No ammonia means no nitrogen cycle.

The tried and true way to do this is by gradually adding fish food into the water. Don’t go crazy and dump an entire jar into your aquarium at once. Instead, add a little bit twice a day in roughly 12-hour intervals.

This fish food will start to break down in the water and slowly increase the ammonia levels. While this is going on you should test the water every day or two to make sure there’s an adequate amount of ammonia in the water (3ppm is the target).

Step 2: Checking The Nitrite Levels

Once your food sabotage has gone on for 7 to 8 days it’s time to see where the nitrite levels are. You’ll keep adding food regardless of what the test turns up, but if you see that there are nitrites in the water then that’s a sign that the nitrogen cycle is underway.

Step 3: Wait For A Nitrite Dip

Continue testing and adding ammonia until your test shows a dip in the nitrites in your tank. This means you’re entering the final phase of the cycle!

Your job now will be to test the nitrite and ammonia levels. Remember, these have to fully disappear before you’re done. Stopping too early can lead to big problems later on down the line.

Once the ammonia and nitrites are fully out of the picture it’s time for you to double-check your nitrate level. Even though this is what we want, too much nitrate can still cause problems for your fish.

Anything over 20ppm can lead to problems for your fish, so perform a water change if this applies to you.

Step 4: Bring In The Critters!

Now that the water is suitable for your new fish it’s time to introduce them to their new home. Start by adding a few hardy fish at a time over the course of a few weeks (depending on how many you have) to play it safe. Adding too many at once can result in an ammonia spike which can be devastating to the life in your tank.

Dropping in everyone at once can result in shifts in water levels and since the tank is in a sensitive state, it’s better to ease into things. You’ve already waited a while for the nitrogen cycle to complete and it would be a shame for something to happen at this stage due to impatience.

Cycling With Fish

While there are plenty of guides out there on how to cycle a tank with fish inside, this isn’t one of them.

Here’s why:

Cycling a tank with fish inside is not ideal at all. Due to the sharp rise in ammonia and nitrites, there’s a very strong possibility that you’ll end up killing some fish. The ones that survive will be subjected to a significant amount of pain and stress.

At Aquarium Source that just isn’t how we operate.

So while we know the steps to take if you want to cycle a tank that’s actively housing fish, we can’t condone it. It just goes against our values when it comes to fishkeeping.

Methods To Shorten The Cycling Process

By now you’ve probably done the math in your head and realized that this entire process could take a while, and you’re not alone. Going through weeks and weeks of water tests and waiting is something that most people would like to avoid.

Fortunately, there are some things you can do to accelerate the nitrogen cycle.

Author Note: Some of these methods are called “seeding” so if you see that used online you now know what they’re referring to.

Use Filter Media

If you have access to filter media from an established tank that can be used to accelerate the tank cycling process. The media from this tank will have already been exposed to the beneficial bacteria we’re looking for and will bring some over to the aquarium you’re cycling.

If possible, using filter media from a tank that’s roughly the same size as the one you’re cycling is ideal. This isn’t a dealbreaker, but it reduces some risk and ensures that the amount of bacteria you’re bringing over is similar to what you’ll need.

Expose Your Filter To An Established Tank

This is similar to the previous method. Simply take the filter you’re going to use in the tank you’re cycling and expose it to the aquarium water in an established aquarium for at least 7 days.

This will give the nitrifying bacteria in the established tank the opportunity to make themselves at home in your new filter. Then, when you bring it back to the new tank you’re cycling this beneficial bacteria will make the journey too!

Start Gardening

Adding some live plants to your tank is another great way to accelerate the nitrogen cycle and manage the amount of ammonia long-term. Some aquarists prefer to add plants that previously were housed in an established aquarium, but new plants are still very helpful.

Plants with a speedy growth rate are ideal for this because if you have to wait weeks for them to grow, you’re not going to get accelerate the nitrogen cycle as much as possible.

Author Note: Be aware that using these methods to accelerate the cycle is not without some risk. When you’re bringing in bacteria from other tanks there’s always a chance that other unwanted hitchhikers could tag along (parasites for example). If you want to play it as safe as possible and have patience, steering clear of these methods is your best move.

Frequent Obstacles When Cycling Your Aquarium

By now you should have a good understanding of fishless cycling and how to manage the process. But that doesn’t mean things can’t go wrong!

Even the most experienced aquarists hit speed bumps when it comes to the nitrogen cycle. It’s just a tricky process.

But there are some issues that will happen far more often than others. In this section we cover them so you’re not lost if they happen to you.

You Can’t Start The Cycle

A common issue that new aquarists face (especially when doing their first fishless cycle) is a lack of ammonia in the tank. Obviously, if you can’t make your ammonia levels rise then the rest of the nitrogen cycle can’t happen!

Fortunately, the solution to this is pretty simple: add more food. Continue with the 12-hour spacing, but drop in more flakes than you were before.

Also, try removing any plants you have in the tank. Their good intentions might be getting rid of all the ammonia! This is the one time where the amazing benefits of live plants in your aquarium can actually be an inconvenience.

There’s No Dip In Ammonia Levels

You have ammonia in your tank. Good. But what if it’s not dropping?

One of the most common causes of this is simply a low pH level. Anything under a pH of 7 makes the nitrogen cycle impossible, so you’ll need to raise it if that’s the case.

If your pH is looking good then make sure you haven’t done one of the other two common mistakes.

The first is using chlorinated water. This will snuff out any of the nitrifying bacteria in your water and leave the ammonia to exist unchecked. Tap water typically has chlorine in it, so that’s something to be aware of.

The other possibility is that you’ve simply been cleaning your aquarium too much. Overcleaning is a great way to remove the bacteria you need for the nitrogen cycle, so take it easy with the scrubbing!

You’re Struggling To Establish Nitrate In Your Tank

Water with chlorine and overcleaning will also impact your ability to facilitate nitrate growth in your tank. Plants can also impact this stage of the cycle as well, but chances are they would have stopped you before you made it to this point.


Learning about the nitrogen cycle and how to cycle a fish tank doesn’t have to be scary. Just like anything, once you understand the process the fear melts away!

If you’re still a little lost we encourage you to read the stage breakdown again. Knowing what’s actually happening on a scientific level will help you carry out the steps of fishless cycling, but also troubleshoot if you run into problems.

If you have any questions about this process we’re more than happy to help. Reach out to us on social or via email and we’ll help you figure out what to do.
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