How to Cycle a New Aquarium: Fishless Method

What do you need before you start a fishless cycle?

You need a running tank, an ammonia source, a dechlorinator and a test kit that reads ammonia, nitrite, nitrate and pH. Fishless cycling is a way of starting the nitrogen cycle, the bacterial process that turns ammonia into nitrite and then nitrate, without putting fish through it; the nitrogen cycle guide explains each stage and why ammonia and nitrite are toxic. The list below sets out what each item does, in the order you will use it.

• Filter and heater, running. The bacteria colonise the filter, and SRAC Publication 4502 allows a modest temperature boost to speed them up.
• Water conditioner. Chlorine kills nitrifying bacteria, so the fill water must be treated first.
• Ammonia source. A measured ammonium chloride solution, or clear household ammonia dosed by calculation.
• Test kit. Ammonia, nitrite and nitrate readings tell you which stage the cycle has reached; pH and KH warn of a stall.
• Optional bacteria starter. A bottled culture that aims to shorten the cycle; see the section on bacteria starters below.

How do you cycle a tank without fish, step by step?

You cycle a tank without fish by dosing ammonia, testing as the bacteria convert it, and finishing with a 24-hour completion test. Each step below is drawn from the published sources in the table above.

1. Fill and dechlorinate. Treat all fill water with a conditioner before anything else goes in; Practical Fishkeeping warns that tap water chlorine kills beneficial bacteria. The guide to treating tap water explains chloramine and why some conditioners also bind ammonia.
2. Run the filter and set the temperature. Keep oxygen up: UF/IFAS advises more aeration if dissolved oxygen falls below 5 mg/L dissolved oxygen (minimum). A small temperature boost helps, but SRAC 4502 limits it to 2–3 °C above operating temperature (max) above the planned operating temperature.
3. Check KH before dosing. Nitrifying bacteria consume alkalinity, and Merck recommends at least 100 mg/L as CaCO3 (minimum) for a healthy biofilter. In soft water, KH can run out during cycling and stall it.
4. Dose ammonia to the target. Dr Tim’s Aquatics recommends 2–3 mg/L ammonia-N; its own solution is dosed at 4 drops per US gallon to reach about 2 ppm ammonia-nitrogen. SRAC 4502 gives 3–5 mg/L total ammonia for aquaculture biofilters, using clear 10% household ammonia at a rate of about 1.6 mg/L rise per 60 mL of 10% household ammonia in 3,785 L (1,000 gal).
5. Test and watch the sequence. Ammonia falls first as the first group of bacteria establishes, then nitrite rises and falls, then nitrate appears. Keep both ammonia and nitrite below 5 mg/L (do not exceed), the level Dr Tim’s warns can inhibit nitrifiers.
6. Run the completion test. Add a full dose and test 24 hours later. Dr Tim’s and Practical Fishkeeping both treat the cycle as complete when that dose is gone, converted to nitrate, with no detectable nitrite.
7. Bring nitrate down, then stock gradually. A finished cycle leaves nitrate behind. Practical Fishkeeping and Aquarium Co-Op aim to keep it below about 40 ppm, so a water change before the first fish makes sense. Merck and UF/IFAS both recommend stocking gradually once the biofilter is established.

How do you know the cycle has worked?

The cycle has worked when a full ammonia dose disappears within 24 hours, nitrite reads zero and nitrate is rising. Reading zero ammonia on one day is not enough, because the nitrite stage can lag well behind. Practical Fishkeeping and Dr Tim’s use the same completion test, which makes it the clearest published standard.

Read the test results as a sequence across the weeks. Rising ammonia with no nitrite means the first group of bacteria has not yet established. Falling ammonia with rising nitrite means the first group is working and the second is lagging. Zero ammonia, zero nitrite and rising nitrate means both groups are working. Kits that cover all three readings are compared in the guide to the best aquarium test kits compared.

How long should it take?

Expect roughly a month to two months. Dr Tim’s gives 30–45 days without its bacterial product, Practical Fishkeeping says 4–6 weeks for most tanks, and UF/IFAS gives 6–8 weeks for a new biofilter to reach full capacity. The shorter figures describe passing the completion test; the longer ones describe a mature biofilter.

Do bacteria starters speed up a fishless cycle?

Bacteria starter makers say their products shorten the cycle, but no source in our fact base independently tests that claim or compares one starter with another. A bacteria starter is a bottled culture of nitrifying bacteria intended to seed a new filter. Fritz Aquatics describes FritzZyme 7 as containing live freshwater-specific nitrifying bacteria and claims it rapidly eliminates toxic ammonia and nitrite; seeds biofilters to shorten cycle time. Dr Tim’s Aquatics designs its ammonium chloride to be used with its own One & Only Nitrifying Bacteria, and its 30–45 days figure is specifically for cycling without that product.

Because those are manufacturer claims, this guide does not name a best bacteria starter. If you use one, the same completion test still applies: a full ammonia dose cleared within 24 hours with no nitrite. A starter that genuinely seeds the filter should reach that point sooner; one that does not will show it in the test results.

Is a tank cycled once ammonia reads zero?

No. Zero ammonia shows only that the first group of bacteria is working. Nitrite-oxidizing bacteria establish later, and nitrite can be toxic from 0.1 mg/L nitrite, according to UF/IFAS and Merck. A tank is ready only when a full ammonia dose clears within 24 hours and nitrite also reads zero. Adding fish at the zero-ammonia stage risks the nitrite poisoning described under new tank syndrome.

A related mistake is reading cloudy water as a sign of failure. A white, milky haze in a cycling tank is usually a bacterial bloom, which Aquarium Co-Op and AquariumScience.org say normally clears in one to two weeks as the filter matures. Both advise against big water changes during the bloom; the guide to cloudy aquarium water explains how to tell a bloom from other causes.

Why does a fishless cycle stall?

A fishless cycle usually stalls because one of the bacteria’s needs is missing: buffer, a safe ammonia level, oxygen or chlorine-free water. Each cause below comes from the published sources.

pH crash from exhausted KH

Practical Fishkeeping links depleted KH during cycling to a pH crash below 6 pH (below), at which point bacterial activity stops. Testing KH and restoring it addresses the cause.

Too much ammonia

Overdosing can happen when ammonia is added by guesswork. Dr Tim’s warns that ammonia or nitrite above 5 mg/L (do not exceed) inhibits nitrifiers, so a partial water change to bring levels down restarts progress.

Chlorine or medication

Untreated top-up water or rinsing media under the tap kills bacteria. UF/IFAS also notes that disease treatments, especially antibiotics, can harm the biofilter.

Low oxygen

Nitrification needs oxygen, and UF/IFAS sets 5 mg/L dissolved oxygen (minimum) as the point to increase aeration.

What should you do after the cycle is complete?

After the cycle completes, lower nitrate with a water change, stock a few fish at a time and start a routine. Merck and UF/IFAS both recommend stocking gradually once the biofilter is established. From then on the fish supply the ammonia, excreted mainly through their gills, and the sources on cloudy water advise against overfeeding; the guide to the best fish food by species type helps match food to the species you add.

From here the job becomes keeping the cycle running: regular partial water changes to remove nitrate and replace KH, and filter cleaning in tank water rather than tap water. The aquarium maintenance schedule guide sets out that routine, and the other chemistry and setup topics are in the water chemistry and setup guides.