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How to cycle a fish tank

How to cycle a fish tank
What is cycling your fish tank?

When we talk about how to cycle a fish tank, we are talking about the nitrogen cycle. This is a natural process that helps to separate toxic waste into something less harmful for your fish.

When you set up a fish tank for your new pet fishes, you will need to create this cycle in their new aquarium. Basically every fish keeper must learn how to cycle a fish tank.

The cycle allows good bacteria to grow in the tank, acting as a natural filter for any contaminants produced by your fish. Their wee and poo have high levels of ammonia which is deadly, the cycling helps convers nitrites into a non-toxic nitrates.

What happens during the cycle?

There are three major stages that occur during the nitrogen cycle:

Ammonia is introduced into the water through waste and raw foods. This is toxic to your fish. Ammonia may be present if the pH levels in your tank water are above 7.

Bacteria begin to form and convert ammonia into nitrites. These are still toxic, but are an important phase in the cycle. You will be able to tell when this happens when the pH levels start to drop below 7 and when the water looks cloudy.

Nitrites form and are converted into nitrates by new bacteria. Nitrates are the end goal of a cycle that filters the wastes in your water. They are not harmful to your fish except in large quantities, so you will need to regularly inspect the water to make sure its levels remain below 20ppm.

How long does it take?

Creating a healthy nitrogen cycle in your new aquarium has no time limit. Usually, however, it will take a few weeks to provide regular water change and carefully check for toxic water. Your pH, ammonia and nitrite in your aquarium water are all good indicators.

What products are needed in cycling the aquarium?


Lighting system



Water test kit

How to cycle a fish tank without fish

If you have never cycled a fish tank before, we have put together a simple step by step guide.

Step 1. Set up your aquarium

Do you know all the equipment you got for your aquarium? Yes, you need to set all that. Heater, filter, air pump, substrate, plants…

see also how to set up a fish tank

Useful bacteria need a place to hold on, namely your substrate and filter sources. In fact, most bacteria will call your filter home.

Once your tank is set up, you want to keep any electrical appliances switched on throughout the cycling process, such as heaters and filters. Doing so encourages beneficial bacteria to grow and can speed up your tank cycle.

Step 2: Check your pH

This is perhaps one of the most common missed steps when it comes to cycling your tank – and the common cause of failed cycles.

You see, cycling can slow down or stop if your pH level drops below 7 degrees. It is always advisable to check your to always check your pH

So, check the water you add to your aquarium with your test kit. If it is less than 7, you need to raise your pH before moving on to the next step.

You see, the beneficial bacteria in your tank release acids that lower the pH of the water over time.

If you notice that your pH levels are below 7, a simple water change of 20% is what is needed to increase the pH and make the cycle go back and forth.

Therefore, be sure to check your pH regularly and adjust it if necessary.

Step 3: Add your ammonia

In a brand new tank, there will be no waste, which means that nothing will break down into ammonia.

You need to know how much water is in your aquarium to add the right amount of ammonia.

So, measure your ammonia…

Aim for 2 ppm for a fish tank that is less than 40 gallons while for a fish tank more than 40 gallons aim for 4 ppm

Less than 40 liters of fish tanks: Strive for 2 ppm

Fish tanks over 40 liters: Strive for 4 ppm

Now you want to make sure you add the right amount of ammonia. And to do this you will use your aquarium test kit, which will give you the result at ppm

But before you do that, you want to allow the ammonia to remain for an hour, so that it is evenly distributed in the water. Next, measure ammonia levels using ammonia tests from your main test kit – Make sure you follow the instructions given in the test kit to get accurate results.

If your ammonia reading is below the above levels, add more ammonia and test again.

If your ammonia levels are high, make a water change. Doing so will replace your high-ammonia water with non-ammonia water, lowering the levels.

Record the amount of ammonia you added. You will need that information in some steps.

Always make sure to check your ammonia levels each day with a test kit. Then wait for your ammonia levels to drop.

This usually takes a week. Once the week is over, it is time to move on to the next step.

Step 4: Bacteria-eating bacteria develop

After a week has passed, it is time to test the nitrites.

If your test comes back well, congratulations – your cycle has officially started!

Now, it is important to remember that these bacteria consume ammonia. The only thing that adds ammonia to this aquarium is you. Therefore, we will have to give the bacteria more food. Add half the amount of ammonia you have added on the first day, but make sure your ammonia levels remain below 5 ppm.

Now what you want to do is monitor your nitrites, check them daily. You should note that nitrite levels continue to rise. Once you see your nitrite levels begin to decline, it is time to move on to the next step.

Step 5: Nitrite-eating bacteria emerge

Now, we still want to make sure that the bacteria have enough food to eat. So, add half a dose of ammonia as needed, each day if you have too, to keep the levels above 1 ppm. When your ammonia and nitrite read 0 in 24 hours, your nitrogen cycle is finished.

Step 6: The Final Test

To make sure your tank is fully cycled, we have to do one last final test…..

When your ammonia and nitrite levels reach zero, add the total amount of Ammonia, the same amount you added on the first day. Now, we will have to wait one last time…

Check back in 24 hours. Check your ammonia and nitrite levels. If they both read zero….

Congratulations! Your fish tank has completed its cycle.

Adding your fish

Now that you can’t find any ammonia or nitrite, it is safe to start adding your fish.

Also, do not simply add a load of fish. You need to do this slowly, do not add more than a few fish at a time. And wait a week or two before introducing more.

You also need to consider cleaning any substrate with a siphon or hose before adding the fish.

Your patience has paid off, and your tank is fully cycled.

If you record the reading of your test kit each day, you will have a clear understanding of the nitrogen cycle.

How to cycle a fish tank with fish

This method is the most common, used by both beginners. Though, this is not the preferred method. You will expose your fish to ammonia and nitrites during the process and most fish will not be able to survive this.

However, some species may be able to survive this better than others. And if you made a mistake by buying your aquarium and fish on the same day, this could the only option to use.

Part 1: Introducing a small scale of fish

You should add about 1-2 fish per 10 liters of water. Adding too much fish will lead to excessive waste. This can create an ammonia spike and kill your fish.

Part 2: Feeding Your Fish

Feed your fish with moderate-sized meals, be careful not to overdo it. As a general rule of thumb feed your fish once every two days. If you overfeed your fish, there may be leftovers, this can decay and increase the level of toxins in your tank.

Part 3: Water Changes

Your fish are exposed to deadly amounts of ammonia and nitrate, so regular water changes are necessary. It will help to ensure that toxicity levels do not increase too much.

Plan to make 10-25% water changes every 2-3 days. Any more than this will be risking removing ammonia and nitrite that is required for the beneficial bacteria to feed on.

Part 4: Testing Toxin Levels

You will need to purchase a test kit to monitor the levels of ammonia and nitrite in your aquarium. Keeping track of the amount of in your tank will help you manage your fish throughout the process.

Nitrate testing should be available. As we discussed, at first, there will be no nitrite in your tank, but after a few days, it will rise.

Testing is important because you will need to know that the levels go down to zero – a signal that the cycling process is over.

Daily testing is recommended.

Part 5: Adding More Fish

You can start adding more fish when the toxin levels have reached zero. This should be done gradually, introducing only one or two fish at a time. After adding, wait about a week and test the water again. If the levels of ammonia and nitrite are low, add more.

It is very important to do this slowly. Adding too much fish will cause the levels of ammonia and nitrite to rise which can cause stress to your fish and lead to disease.

What happens if you do not cycle your aquarium?

The nitrogen cycle is funny because it will happen whether you want it or not.

If you put your fish in an un-cycled aquarium, food and waste will release ammonia, which begins the cycling process.

Just one problem…

You see, your fish are in the tank while the toxic levels of ammonia and nitrite increase. To say that these things are harmful to your fish is an understatement.

This toxic environment can be harsh for your fish – most fish cannot survive this cycle, and those that do are at risk of infection and do not last long.

So, while the tank can cycle itself without your efforts, there is no guarantee that your brand new fish will survive.

Why is it important to cycle your fish tank?

In the wild, fish do not have the same problem of high toxicity in the sea as because of the abundant water around them. When fishes are being put in an aquarium, the enclosure may indicate that they are full of toxins if the tank is not properly cycled. It is very important to rotate your tank to keep your fish alive and healthy.

Unfortunately, if you have already bought a fish together with your new tank, the only option may be to cycle with your fish in the tank.

How to cycle a fish tank fast

Yes, the nitrogen cycle can be accelerated. If you do not want to wait 6-8 weeks, you can use these methods.

You will need to access an established tank to use most of them.

Adding a Media Filter From an established tank

The media filter in the established tank will contain the nitrifying bacterial bacteria. You will not have to wait for the bacteria to start growing naturally, so your cycling can be faster.

Adding Gravel From an established tank

If you have access to an established tank that uses an under gravel filter, bacteria will be attached to the gravel. It will have the same effect as when using a media filter.

Take about a handful of gravel and hang it in a mesh bag in your filter, if you can. If not, and using an under gravel filter, place it on top of the gravel in your new tank.

Use Live Plants

Instead of setting up an empty tank with very small fish, you can quickly add live aquarium plants and focus on growing them with good lighting, substrate, and fertilizer. Especially, if you find them in an established tank. Plants use a process called protein synthesis to measure the levels of ammonia in your tank.

As soon as the plants (or algae) show new growth, the cycle is over. Your plants successfully utilize ammonia and nitrate and convert them into new leaves and roots. Gradually add a few fish, and then use a water test kit to make sure the ammonia and nitrites are 0 ppm and the nitrates are less than 40 ppm.

This is important, the bad side of this can be that you can pass on some bad things from that established tank which may contain harmful bacteria. So be careful, never to move anything from a tank that is known to be contaminated by harmful substances.

What if I Still Have Problems?

If, after six to eight weeks of cycling, your levels of ammonia and nitrite are unsatisfactory, it may be due to these;

  1. Was chlorine or chloramine removed from the water added to the tank. If chlorine was in your tank it is likely to kill the bacteria that were trying to start the filter. Or the ammonia in chloramine can be more than your new colony of bacteria.
  2. Do you regularly make water changes? This will remove excess waste before it kills the fish or beneficial bacteria you are trying to grow.

Knowing how to cycle a fish tank requires a bit of effort (and patience) on your part, but the results are perfectly worth it. By preparing an environment for the reception of your new fish, you greatly reduce the loss of life and make your aquarium maintenance process easier.





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