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Cycling Fish Tank – All You Need to Know


The Nitrogen Cycle

The Nitrogen Cycle in a Fish Tank
To understand why cycling a fish tank is so important, you have to have a basic understanding of the nitrogen cycle. Because an aquarium is such a small

This cycle is completely natural but it needs a little help getting started. All you have to do is make sure all of the components are in place and wait.

You might be waiting for quite a while, too. It can take as long as three months for the nitrogen cycle to get established in a new aquarium. You can’t just come home from the pet store, fill the tank with water, and add the fish the same day.

Here’s how the nitrogen cycle works. Nitrogen converting bacteria naturally grow in the tank, primarily in the substrate and biofiltration media in the filter. Adding a hardy fish or two helps this process along because the bacteria need detritus to form and flourish, but there are non-fish alternatives, too. More on that later.

As fish create waste, the bacteria convert it into ammonia. Ammonia is toxic to fish, and if enough of it builds up in the water, the fish won’t survive. In the first step of the nitrogen cycle, nitrogen-fixing bacteria break down the ammonia, turning it into nitrite.

Nitrites are also really deadly for fish, so the nitrite level has to be monitored carefully. If the cycle is working correctly, the beneficial bacteria continues to do its work, breaking down the nitrite even further into nitrate, which is mostly harmless to your fish.

Now that you have a basic idea of how important the nitrogen cycle is, let’s take a look at how you can get it started the right way in your tank.

How to Successfully Cycle a Fish Tank?

One of the most important supplies you’ll need when cycling a fish tank is an aquarium test kit. This is the only way to make sure that things are progressing. All you have to do is dip a test strip into the aquarium, wait for it to develop, then compare the results to the chart on the side of the bottle.

These kits tell you how much ammonia, nitrites, and nitrates are the water. This gives you a good idea of whether or not an adequate number of beneficial bacterial colonies have grown in your tank. It will also tell you when it’s finally safe to add fish.

Cycling with Fish

As we mentioned, to get things started, you have to add a few fish to the tank before it is cycled. This is a natural way to introduce ammonia to the tank, which is needed to start the cycle. That said, some people find this method to be unnecessarily cruel to the fish and prefer a fishless cycling approach, which we describe below.

If you choose to do cycling with fish, a good rule of thumb is to add 1.5 fish for every 10 gallons of water in the tank. So, for a 10-gallon tank, add two fish. For a 20-gallon tank, add three.

You have to choose these fish carefully because they have to be tough enough to handle the elevated ammonia and nitrite that is expected at the beginning of the cycle. Barbs, tetras, goldfish, and Bettas are generally pretty good picks.

Adding too many fish at this point will overwhelm the tank, causing the ammonia levels to spike. This is a common reason why a lot of new aquarium owners find their fish all die within the first few weeks.

Fishless Cycling

Fishless cycling is becoming more and more popular because it’s an easy way to kick off the nitrogen cycle without risking the health of any fish. If you prefer this approach, all you have to do is add a little bit of ammonia to the tank when there isn’t any fish in it to encourage bacterial growth.

Add a small amount to the tank every few days, being sure to check the water so you know when the cycle is in motion.

How to Know When a Tank Is Cycled

There’s no way to predict how long it will take to fully cycle a tank. It depends on a lot of factors, including how many gallons the tank is, what kind of substrate and filter media are being used, and whether or not there was enough ammonia present to get the process going as quickly as possible.

Expect fish tank cycling to take anywhere from two to eight weeks. If you do everything right and have a bit of luck, you might end up on the short side of this range. Check the water after two weeks to evaluate where you stand.

The nitrite should spike around the second week of cycling. This indicates that whatever ammonia is in the water is being successfully broken down by the bacteria. Some people might see this spike as a bad thing, but at this point in the process, you want this spike. It shows that the cycle is progressing.

After the nitrite spike, recheck the water every week until the nitrite level falls back to zero. This means that the bacteria are again doing their job, converting the nitrites into nitrates. At this point, you can safely begin to add fish.

Adding Fish

It is important not to add too many fish at once or you will overwhelm the cycle and the bacteria will not be able to keep up. Remember, the fish create the waste that causes the ammonia to spike, so too many fish at once will result in too much ammonia.

After about eight weeks have gone by, you can safely add more sensitive fish. Keep checking the water every week, though, to make sure there aren’t any ammonia or nitrite spikes after adding more fish.

Helpful Hints

Here are some things to remember to make cycling your fish tank a little easier:


Don’t worry about the ammonia when you’re starting the cycle. Some people check their tank water too frequently and panic if they see an ammonia spike. This is what you want! The bacteria need this spike. It’s what they feed on to get the cycle started.


If you opt to cycle with fish, don’t be surprised if you lose one or two in the first few weeks. It’s hard to predict how fast the ammonia will rise and how quickly the bacteria will break it down. It might be too much for your fish.


Fishless cycling is considered more humane and is a quick and easy way to control the start of the cycle.


Don’t get ahead of the cycle and add more fish before the tank is ready. Seeing the nitrites drop is an encouraging sign that things are going well, but adding too many fish too soon will overwhelm the bacteria and the ammonia will build up all over again.


You may see a bacteria bloom in the first few days. In a bacteria bloom, the water gets cloudy and you might start to wonder if something is wrong. This is actually a good thing because it signals bacteria growth. The water should clear again without any intervention in a day or two.


If you’re setting up a second aquarium in your home, a quick way to cycle a second tank is to take some of the substrate or filter media from the first tank and add it to the second. This gives you a bit of a head start on bacterial growth and can cut down cycle time immensely.


Adding plants during tank cycling is recommended and can help the process along. Plants absorb excess nitrates which leave the water healthier at the end of the cycle. Plants are not affected by ammonia. That said, dead or floating leave can increase the ammonia so be sure to remove any as soon as you see them.

Don’t Rush

It’s hard to wait up to eight weeks for a new tank to be ready, but it’s worth it in the long run. If you’ve ever set up an aquarium and wondered why all of your fish died in the first few weeks, it’s likely you didn’t wait long enough for the cycle to establish itself.

Starting off on the right foot with a new tank is important. Establishing adequate and healthy bacteria colonies in your tank helps prevent problems in the future. Taking your time and getting it right at the beginning can save you from chasing fixes in the future.

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