How to Cycle a Fish Tank (The Right Way)

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After setting up your fish tank and filling it with water the next step is to cycle it.

But the question is, how to cycle a fish tank?

You cannot add the fish straight away. If you do then they will die because the water is too sterile and the good bacteria has not yet been established.

Fish in the wild live in an ecosystem containing plants, microorganisms and contaminates.

A fish tank is an enclosed ecosystem so when you set up a new fish tank then you need to cycle the tank to create a healthy and thriving environment for your fish to live in.

When you are first starting out then the nitrogen cycle can seem confusing but it does not need to be this way.

I will be showing you, using the fishless cycle, how to do it the right way. It is easier than you think.

Please do not use fish to start the nitrogen cycle. You can start the cycle by adding a pinch of fish food to the tank on a daily basis.

This is a lot less cruel then subjecting a poor fish to the toxins in your tank.

What is the Nitrogen Cycle and Why is it So Important?

The nitrogen cycle is also called the break-in cycle, biological cycle or the nitrification process. They all refer to the same thing – the nitrogen cycle.

Why is this process so important?

Fish pee and poop in the same water that they swim around in. Uneaten food starts to decay and rotten plants add to the waste.

These waste products break down and release ammonia into the water.

Ammonia is deadly to fish so good bacteria is needed to eat ammonia and keep it under control.

The nitrogen cycle is a three stage process that in the end produce two beneficial bacteria that consumes toxic ammonia and nitrites.

How to cycle a fish tank

Stage 1 – The Ammonia Stage of How to Cycle a Fish Tank

Everything starts with the waste in the fish tank.

Uneaten fish food, rotting plants, pee and poop break down and release ammonia into the water.

This ammonia can reach deadly levels unless beneficial bacteria start to form to eat the ammonia.

Beneficial bacteria appear naturally on their own and once they have multiplied enough, they will eat ammonia as soon as it appears.

Stage one is complete when the ammonia levels begin to decline. Your tank will then enter into the second stage of the nitrogen cycle.

Stage 2 – Nitrites During the Cycling Phase

The bacteria that eats the ammonia give off a new chemical – nitrite.

Nitrite is also harmful to fish and is highly toxic to them.

But as the nitrite levels rise then a second bacteria appears in the fish tank.

This second bacteria eats nitrites and as before it grows in number and eats the nitrites as quickly as they are produced.

When the nitrite levels begin to fall then the fish tank is entering the final stage of the nitrogen cycle.

Stage 3 – Nitrates Signalling the End of the Cycling Process

The second bacteria that eat the nitrites release a new chemical called nitrate.

Nitrates, in small amounts, are harmless to your fish.

This chemical is the final stage of the nitrogen cycle and is an indication that the initial cycle has been completed.

You will need to purchase a test kit to measure the ammonia, nitrites and nitrates in the water.

Once the ammonia and nitrites reaches 0ppm then the cycle is complete.

Too many nitrates can be harmful but this is easily kept under control by performing regular water changes.

Nitrites and nitrates

Conclusion on How to Cycle a Fish Tank

That is how simple the whole process is.

The purpose of cycling your fish tank is so that the two beneficial bacteria multiply enough to consume the ammonia and nitrites that are so harmful to your fish.

Once the tank is fully cycled then you can start adding a few fish. Only add a few fish then wait two weeks before adding more.

This is so that you do not overwhelm the established bacteria by adding too many fish in one go.

The nitrogen cycle is never ending. It is constantly happening so that your fish stay safe.

Hopefully you agree that you do need to cycle a fish tank before adding fish.


How long does it take for a fish tank to cycle?

It should take between six to eight weeks.

What is the fastest way to cycle a new tank?

If you already have an established tank then take some filter material, gravel, plants and decor from the existing tank and place them in the new tank. You should then be able to add fish in a couple of days.

Will my fish die if I don’t cycle the tank?

Ammonia, which is the toxin created by waste, will build up and cause disease and then death in your fish.

How do I know when my aquarium has cycled?

Your tank will be cycled once the levels of nitrite and ammonia reach 0ppm (parts per million).

Can I add fish after 24 hours?

It is not recommended that you do that. Let the cycle complete itself and then add fish.

Does Brown algae mean my tank is cycled?

You can see brown algae forming during the cycling phase but it can also appear in a well established fish tank. So, it is not an indication that your tank is cycled.

Can you put fish in a new tank the same day?

No, please do not do that. If the tank has not been given a chance to go through the cycling process then your fish will die.

Will live plants help cycle my tank?

Live plants do absorb ammonia, nitrites and nitrates so if you add them when you set up the tank then the tank will cycle faster.

Will snails keep a tank cycled?

They will to a certain extent as they produce waste but you will need quite a few snails.

How do I keep my hospital tank cycled?

The best way is to always have a small sponge filter running in your main tank. Then when you need to use the hospital tank, transfer the sponge filter with the fish to the hospital tank.

Should you do water changes when cycling a tank?

Performing frequent small water changes in a cycling aquarium will help to reduce ammonia and slightly speed up the nitrogen cycle.

Can you cycle a tank with shrimp?

No, you cannot.

Does algae help cycle a tank?

Is is quite common during the cycling phase for your tank to experience an algae bloom. This is actually a good sign and is an indication that your tank is almost fully cycled.

Photo of author


Irma Bense is the founder of She has over 42 years of experience in keeping fish. She has kept both freshwater and saltwater fish through the years. She has extensive knowledge in breeding numerous species of fish from Show Guppies, Cory Catfish, German Blue Rams to Apistogrammas. You can learn more about Irma on the 'About' page.

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