Cycling Aquarium Water

In simple terms, to cycle an aquarium means to establish a bacteria colony inside the filter to ensure that it is capable of breaking down harmful fish waste - ammonia.

A filtration system is a key component of life support in your fish tank or pond. You are relying on your filter to keep your water clean and fish healthy.

There are two ways to effectively cycle a fish tank. Using a small colony of fish, or the fishless cycle.

Cycling a Tank with Fish

A good way to understand how filter bacteria establishes, is to imagine what would happen to your bathroom toilet if you left it uncleaned for a few weeks. Think of the amount of bacteria that would build-up on the pan!

Cycling a fish tank is practically the same process, because you are relying on the fishes toilet habits to encourage bacteria to grow in your filter. When fish go to the toilet, they produce ammonia.

The bacteria that develops on your filter will feed on ammonia to survive, reducing the levels present in your water.

If using this method, it is important to use only a small colony of hardy fish, that will not turn belly-up when exposed to moderate ammonia. It is important not to add too many fish, as this may cause harmful levels of ammonia to form, or "spike", and could be fatal to your fish.


Fish commonly used to cycle a fish tank include hardier species such as cherry and tiger barbs, neon tetra and guppies.

The Fishless Cycle

The most common method used when establishing a new tank, particularly by beginners and those new to the hobby, is the fishless cycle.


This is where you manually add ammonia and/or filtration bacteria, to help "start" your aquarium filter. It does not require the use of live fish.


To perform a fishless cycle, fill your aquarium with water, power-up the filter and add filter starting bacteria. You can find examples of filter-starting bacteria via this link.

There are many "quick start" treatments out there that will speed-up the cycling of a new tank, which is handy if you need to setup an aquarium in a hurry or emergency, but it is recommended that you allow as much time as possibile to create a suitable habitat for you fish.


Patience is key to the effective cycling of a fish tank. Cycling a coldwater or tropical aquarium usually takes around 5-7 days.  Marine aquariums can take quite a bit longer!

It is very wise to test the level of ammonia present in your tank before adding fish, even if you have completed a period of cycling. You can browse our test kits via this link.

Ammonia can also be produced by rotting plant matter, dead livestock and uneaten food. Take care to manage these aspects responsibly.

Recommendations Remember that Ammonia and Nitrite can be lethal to livestock. Always allow as much time as possible to cycle your aquarium - ideally 5-7 days or longer. Don't be in a rush to add too many fish at once - you do not want to overload the filter with ammonia. Buy a test kit to monitor ammonia levels click here. If your ammonia levels are high, it may suggest that you have too many fish or an ineffective filter. Use a regular supplement of filter bacteria to cultivate your filter colony click here. A 10-20% water change is recommmended each week to help reduce nitrate.

The Nitrogen Cycle

This helpful diagram will show you how ammonia is processed through your filter:


Harmful ammonia is first broken down into nitrite, and further processed into nitrate. A good test kit will read the levels of ammonia, nitrite and nitrate in your aquarium water.

Ammonia and nitrite can be potentially lethal to your fish. You will want to keep these levels low. Nitrate is less harmful to your fish, and can be absorbed by live plants as an effective fertiliser. Nitrate is diluted and reduced by performing regular water changes.

It is good for all repsonsible fish keepers to know about the nitrogen cycle as it can give you a better understanding of whats going on inside the fish tank, and how to potentially resolve any problems that arise.

Keeping your aquarium well oxygenated will also help the breakdown of ammonia. You can find air pump and airstone kits via this link.

If you have any questions about cycling your aquarium, please contact us.