Why Do We Have To Cycle An Aquarium? (Finally Answered!)

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It is important to cycle a new aquarium to make it safe for fish to live in. Cycling an aquarium is the process of allowing bacteria to colonize the filter in a new aquarium in preparation for fish to be added. There are a number of ways to cycle an aquarium including the ‘fish in’ cycle, the ‘fishless’ cycle, and using ‘live bacteria in a bottle’ to start the cycle.

I first started keeping fish in the 1990s and the advice for new fish keepers was just leave your tank for a few days, then add fish. I don’t remember anyone ever discussing ‘cycling’ a tank, no one ever mentioned the ‘Nitrogen Cycle’ and there was no internet to refer to. We just had to set up a tank and work the rest out for ourselves.

If there is one topic that has been hotly debated on every fish-keeping forum over the last few years it is WHY do we have to cycle an aquarium?.

Why Do We Have To Cycle An Aquarium

We have to ‘cycle’ an aquarium to make it safe for fish to live in. What this means, in reality, is we have to allow time for enough bacteria to colonize our aquarium filters to be able to deal with the waste from our fish.

Cycling is just the time it takes for there to be enough bacteria to process the fish’s waste.

If we fail to cycle our aquariums properly, when we add fish and they begin to excrete waste (ammonia) there may not be sufficient bacteria in the filter to be able to convert that waste from the highly toxic ammonia to the far less toxic nitrate. Bacteria in the filter converting ammonia to nitrate is known as the Nitrogen Cycle.

What Is The Nitrogen Cycle?

According to Wikipedia, the nitrogen cycle is ’the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmosphere, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biological and physical processes.’

To be fair, that doesn’t mean a great deal to me, so let’s look at the nitrogen cycle in a fish tank.

When our fish go to the bathroom, they essentially excrete ammonia. This ammonia is extremely toxic to fish, but luckily there is a bacteria that lives in our aquariums, and especially our aquarium filters, which converts the ammonia to nitrite. This bacteria is called Nitrosomonas Bacteria.

Unfortunately, nitrite is also toxic to fish, although less so than ammonia. We have to rely on another bacteria, which also lives in our filters called Nitrobacter Bacteria. Nitrobacter Bacteria converts nitrite to nitrate. Nitrate is far less toxic to fish, but only in low concentrations.

This diagram is courtesy of James Milner of the aquarium.club demonstrates the nitrogen cycle in an aquatic ecosystem.

3 Main Stages Of The Nitrogen CycleAmmonia (Fish Waste) – Extremely Toxic to FishNitrite – Very Toxic to FishNitrate – Only toxic to fish in high quantities


When our fish go to the bathroom, their waste is very high in ammonia. Ammonia is extremely toxic to fish and aquatic life. Without filtration, our fish will quickly be poisoned by ammonia. Their gills will burn and they will struggle to breathe.

Ammonia is converted to Nitrite by Nitrosomonas Bacteria.

Nitrosomonas bacteria are present in our aquariums. Nitrosomonas bacteria colonize all surfaces of our aquarium equally. They colonize the gravel, rocks, internal decoration etc. Nitrosomonas Bacteria love oxygen-rich water, which is why they do so well in our filters. Whatever type of filter you are using, works because water is constantly being passed over or through the filter media, supplying the Nitrosomonas Bacteria with oxygen.

We should all aim to make sure our aquarium has 0 ppm (parts per million) of ammonia when we test with an aquarium test kit.


Nitrite is less toxic to fish than ammonia, but it can still kill the fish very quickly if levels are allowed to build up.

Nitrite is converted to Nitrate by Nitrobacter Bacteria.

Like Nitrosomonas Bacteria, Nitrobacter Bacteria colonize every surface in the aquarium equally and they also love to live in the oxygen-rich environment of our filters.

We should aim for our aquariums to have 0 ppm (parts per million) of nitrite


Nitrate is much less toxic than ammonia or nitrite. Fish can tolerate fairly high levels of nitrates, although it does vary by species. Some of the more delicate fish demand 0 ppm nitrate too.

Regular aquarium filters DO NOT remove nitrate from aquariums. There are specialist filters on the market which remove nitrate, but it is extremely rare for hobbyists to use them due to their cost and size. There are other techniques to remove nitrate, but they are beyond the scope of this article.

To remove nitrate from our aquariums we have to carry out water changes. Personally, I like to change between 10% and 25% of the water in each of my aquariums every week. How much water I change very much depends on which species is in the particular aquarium.

How Do You Cycle An Aquarium?

I have written an extensive article titled How to Cycle A New Aquarium which looks at the subject in greater detail, but below I will give a brief summary of how to cycle an aquarium.

There are essentially 4 ways to cycle an aquarium;

  1. Fishless Cycling
  2. Fish-in Cycling
  3. Pre-cycled media
  4. Live bacteria in a bottle

Fishless Cycling

Fishless cycling involves us aquarists pouring liquid ammonia into our fish tanks. By adding a little each day, we are ‘feeding’ the bacteria in our filters allowing them to multiply to the point where we could add fish.

Fish-in Cycling

The fish-in method involves taking a very hardy species of fish, like a Zebra Danio and adding a small group of them to a new tank. Zebra Danios are extremely hardy and can ride out the ‘spike’ of ammonia and then the spike of nitrite in the aquarium whilst the bacteria in the filter is establishing.

Pre-Cycled Media

If you own more than one aquarium, you can start the cycling process by moving some of the media from one filter into the filter on your new aquarium. In theory, if you move some media from an established tank into the new tank, you could add the fish straight away, although I would still recommend adding just a few fish at a time so the filter doesn’t become overwhelmed.

Live Bacteria in a Bottle

There are now a number of different brands of ‘live bacteria’ that we can buy either online or from a local fish store. The theory is, pour the live bacteria into the aquarium, then add the fish. To be honest, I am extremely skeptical about using live bacteria and I would not recommend it as a way to cycle an aquarium.

How Long Does It Take To Cycle An Aquarium?

This is very much like asking ‘How long is a piece of string?’. The answer is, it depends. It is not possible to say how long an individual tank will take because there are so many variable factors.

The only way to know when an aquarium is cycled is to use a good-quality aquarium test kit. I have always had good success with the Master Test Kit from API. Once your test kit starts to show nitrate is present in the water, you know your filter is converting waste from ammonia to nitrite, then to nitrate and you are ready to add fish.

In Conclusion

Cycling an aquarium is undoubtedly the most important part of setting up a new tank. If we just fill our tanks with water and chuck the fish in on day one, there is a very good chance the fish will quickly be overcome by the ammonia and they will die.

About the Author

I’ve been keeping, breeding, and showing tropical fish for nearly 30 years. Over that time I’ve done it all! I’ve had great success and I’ve made some really foolish mistakes (like the time I bought an Asain Walking Catfish). Read more…
Richard James

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