How to Cycle a Fish Tank: Nitrogen Cycle, Test Kits and Conditioners

Cycling grows the bacteria that turn toxic ammonia from fish waste into nitrite and then less harmful nitrate. VCA says a new tank needs 4 to 6 weeks to cycle before fish go in, and UF/IFAS puts a new biofilter at 6 to 8 weeks. A liquid test kit shows when ammonia and nitrite reach zero.

The nitrogen cycle in one paragraph

Fish waste, leftover food and decaying matter release ammonia. One group of bacteria turns ammonia into nitrite, and another turns nitrite into nitrate, which Aqueon describes as eventually becoming nitrogen gas. Ammonia and nitrite are the dangerous ones: RSPCA Australia says ammonia is toxic to fish from 0.1 ppm, and UF/IFAS says nitrite can be toxic at 0.10 mg/L. Nitrate builds up more slowly and is removed with water changes.

A new tank has none of these bacteria. They grow on the filter media and gravel over several weeks, which is what “cycling” means.

How long does cycling take?

The sources disagree, so here they are side by side:

We follow the veterinary and extension advice: plan on four weeks or more, and let the test kit decide.

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Fishless cycling, step by step

Fishless cycling feeds the bacteria with ammonia instead of fish waste, so no fish are exposed. Dr. Tim’s Aquatics, which makes ammonium chloride for this, describes the steps:

1. Set up the tank with filter and heater running and the water treated with conditioner.
2. Add ammonia to about 2 to 3 ppm. Never go above 5 ppm.
3. Test ammonia and nitrite every 2 or 3 days. Ammonia falls first, then nitrite rises and falls.
4. Top up with a small dose of ammonia every few days to keep the bacteria fed.
5. The tank is cycled when a full dose disappears overnight to nitrate, with no nitrite showing.

If the pH drops too low, Dr. Tim’s says the cycle can stall; a partial water change brings it back up. Add fish a few at a time afterward, as RSPCA Australia advises, so the bacteria can keep up.

Test kits: what to measure

A freshwater test kit measures ammonia, nitrite, nitrate and pH. API’s Freshwater Master Test Kit, for example, is a liquid kit with four test tubes and a color card, covering pH, high range pH, ammonia, nitrite and nitrate. Dip-in test strips are the other common format.

Seachem notes one catch: some ammonia test kits read wrong after Prime is added. Nessler-type kits show an off-scale brown, and salicylate kits must be read right away. Read your kit’s instructions before relying on a result. Once the tank is established, RSPCA Australia says testing every one to two weeks is enough.

Water conditioners: what they do and do not do

VCA says most tap water contains chlorine, so it must be treated with a water conditioner; RSPCA Australia adds chloramine. Use conditioner on every fill and top-up.

Some conditioners claim more. Seachem’s label for Prime says it removes chlorine and chloramine and detoxifies ammonia, nitrite and nitrate, binding them for up to 48 hours, at 5 mL per 50 gallons of new water. That can help in an emergency, but it does not grow bacteria. Bacteria starter products, such as those Aqueon and Dr. Tim’s sell, claim to speed cycling; we describe them only by their labels, and the test kit still decides.

After the cycle

UF/IFAS advises testing ammonia weekly in fish production systems and keeping dissolved oxygen above 5 mg/L. For routine care, see how to clean a fish tank, which keeps the bacteria while removing nitrate. The filters chapter shows how to clean media without killing the colony, and the setup checklist lists what else you need. If fish show signs of illness, see a veterinarian who treats fish.