How to cycle a new aquarium (and why your fish depend on it)

Short answer: cycling a tank means growing colonies of beneficial bacteria that turn toxic fish waste into something far less harmful, before any fish live in the water. Fish constantly produce ammonia, which is poisonous. One group of bacteria converts ammonia into nitrite, which is also poisonous, and a second group converts nitrite into nitrate, which is much safer and is removed by water changes. Until both colonies are established, a tank cannot safely hold fish. Done without fish, cycling usually takes about two to six weeks, and a test kit tells you when it is finished.

The nitrogen cycle in plain English

Every fish, and every scrap of uneaten food, releases ammonia into the water. In an established tank you never see a problem because bacteria living in the filter eat that ammonia as fast as it appears. The chain looks like this:

• Ammonia — produced by fish and decaying food. Toxic even in small amounts.
• Nitrite — the first bacteria turn ammonia into nitrite. Still toxic.
• Nitrate — a second group turns nitrite into nitrate. Far less harmful, and you export it with regular water changes.

Cycling simply means growing both groups of bacteria until they can keep up. They live mostly in your filter media, not in the water itself, which is why you protect that media when you clean it.

Fishless cycling, step by step

The kindest method is to cycle before you buy fish, so nothing has to suffer through the process.

• Set up the tank, add dechlorinator to the fresh tap water, and run the filter and heater.
• Add a source of ammonia — either a bottled ammonia product or a small pinch of fish food left to break down — to give the bacteria something to eat.
• Test the water every couple of days with a liquid test kit. You will see ammonia rise, then nitrite rise as the first bacteria arrive, then nitrate appear as the second group establishes.
• The tank is cycled when both ammonia and nitrite read zero within 24 hours of adding ammonia, and nitrate is present.

Warm water and existing bacteria both speed things up. A bottled culture such as Seachem Stability seeds the tank with the right organisms, and a squeeze from the filter of an established tank works too. Ask us in the shop — we can often help a local tank along.

Why a test kit is the whole point

You cannot see ammonia or nitrite. Without testing you are guessing, and guessing is how fish get hurt. A liquid kit that reads ammonia, nitrite, nitrate and pH is the one tool we would never skip. Learn the readings during your fishless cycle and they will make sense forever after.

Cycling with fish already in the tank

If you already have fish in an uncycled tank, you can still get through it, but you have to keep ammonia and nitrite low by hand. Test daily and do a water change whenever either one climbs. A conditioner that temporarily detoxifies ammonia, such as Seachem Prime, buys time between changes. Go slowly, and do not add more fish until the readings hold at zero.

How long, and what is normal

Two to six weeks is typical. A brief cloudy patch in the first days is common and usually a harmless bacterial bloom. Resist the urge to rush — the wait now prevents the far more upsetting losses later.

Once your readings are steady, plan your stocking with how many fish can I keep, and if you are still gathering gear, start with your first aquarium.

Written by the team at Tropical Fish Imports in Lethbridge, Alberta. Last reviewed August 2026.

Stuck partway through a cycle, or not sure how to read your results? Bring your numbers in or message us — we help local fishkeepers work through this every week.