What is aquarium cycling?

Cycling is the establishment of a bacteria colony inside the filter, gravel, and even decorations. These colonies transform ammonia, produced by fish waste and decaying food, into nitrite, and then nitrate. These are far less dangerous and are removed through water changes. This process is known as the nitrogen cycle, and every new aquarium must go through it.

Getting started

Begin the process by filling the aquarium with tap water, adding a dechlorinator to remove chlorine which is harmful to fish. Fit your filter into the aquarium and let it run continuously. Even though you are not yet stocking with fish, the filter needs to run as it provides the main home for these bacteria colonies. 
You may also add an air pump and an air stone at this point to create oxygen-rich water, which speeds up bacterial growth.

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Two methods of cycling your tank

For beginners, we recommend carrying out the fishless cycling method. This includes adding a small dose of bottled ammonia or small amount of fish food each day to feed the developing bacteria colonies. You can also add live-bacteria to shorten this process.
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Aquarium fish flakes in front of a fish tank.
Ammonia can be introduced into your tank with a small amount of fish food

Another method that you can follow is fish-in cycling using a few hardy species, such as zebra danios to produce natural ammonia. If you follow this method, be sure to test the water daily and change the water often to keep the fish safe.
Browse our selection of test kits for safe cycling.

The nitrogen cycle

At first, you will see a rise in ammonia. Soon, nitrite will appear on your test as the first groups of bacteria begin to convert the ammonia. Finally, the bacteria turns the nitrite into nitrate. When both the ammonia and nitrite levels consistently read zero, and nitrate levels start to ascend, your tank is fully cycled and ready for fish.

Illustration of the nitrogen cycle in an aquarium
The nitrogen cycle keeps your aquarium safe for aquatic life

Adding fish to your tank

Even though your tank is fully cycled, stock fish slowly. A sudden increase in waste may overwhelm the bacteria and cause a dangerous spike. Introduce a few fish at a time, testing the water regularly afterwards. Be sure to perform partial water changes when required.

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Troubleshooting

In this section, we will cover a range of issues and concerns you may observe during the cycling process:

My ammonia levels are high, what do I do?

High ammonia during cycling is normal, but if you already have fish in the tank, perform a 50% water change. Stop feeding for a day and add live bacteria to help the filter mature. Make sure that the filter is running consistently.

The nitrite levels are rising, is my tank ready?

Rising nitrite means that the bacteria has converted ammonia, but hasn't fully matured. Keep testing daily, and conduct partial water changes only if fish are already in your aquarium. Once your nitrite drops to zero, your tank is nearly done cycling.

How do I know when the cycle is complete?

Aquarium cycling has completed when ammonia and nitrite consistently read zero, and your nitrate levels begin to rise. At this point, your tank is safe for introducing fish.

Testing aquarium tank water for nitrite and ammonia levels
Daily testing is important to make sure your nitrite and ammonia levels are consistently 0

Why is my aquarium water cloudy?

Cloudiness can be caused by a bacterial bloom, which is a normal part of the cycling process. Avoid cleaning your filter media, and allow the bacteria to naturally settle. The cloudiness will usually clear within a few days.

Can I add all my fish at once?

Even a fully cycled tank can be overwhelmed by the waste. Be sure to add fish gradually, monitor the ammonia and nitrite levels with a test kit, and conduct water changes.

What do I do if my fish are stressed during cycling?

Signs of stress may include gasping at the surface, lethargy, or clamped fins. Start with a partial water change and ensure your filter is working correctly. Reduce feeding until the water parameters stabilise.

Zebra danio fish in a planted aquarium
Zebra Danios are hardy fish suitable for fish-in cycling

Ready to start your aquarium cycle?

Cycling typically tales four to six weeks, but this time may differ for each aquarium. Even after the cycle is complete, regular testing is important to protect your fish. 

Following these steps, you'll be able to create a healthy and balanced ecosystem where your fish can thrive for years to come.

Check out our full range of aquarium products today to get you started with all the treatments and equipment you'll need.