Fishless cycle ammonia calculator
Cycling a new tank the safe way, with no fish? Work out how much household ammonia to add to hit your target level, then test and top up from there.
Use only plain ammonia — no detergents, scents or colour (shake it: it should not foam). Most people cycle to 2 ppm. Bottle strength is often unprinted, so treat this as a starting dose: add, test in an hour, and top up toward your target.
How the dose is worked out
To raise the tank by a given ppm (milligrams per litre) you need that many milligrams of ammonia for every litre of water. Household ammonia is a solution of ammonia in water at some percent by weight, so the calculator converts your target and tank size into the millilitres of that solution to add.
Why it is a starting dose, not a prescription
The catch: most household ammonia does not print its concentration, and strength varies by brand. So treat the result as a first pour: add it, wait an hour, and test with a liquid test kit. If you are short of your target, top up; if you overshot, a partial water change brings it down.
What ammonia to use
Use only plain ammonia — no detergents, surfactants, scents or colour. Shake the bottle: if it foams, it has additives and is not suitable. Most keepers cycle to about 2 ppm and hold it there.
Reading the cycle
Keep re-dosing to your target while the bacteria establish. When the tank can clear both ammonia and nitrite to zero within 24 hours, it is cycled. The full method is in our fishless cycling guide.
Source
- mL = (target ppm × litres) ÷ (density × percent × 10), assuming aqueous-ammonia density ≈ 0.96 g/mL. Bottle concentration varies, so the figure is a starting estimate to test-and-adjust.
Common questions
How much ammonia do I add for a fishless cycle?
Enough to reach about 2 ppm; for a 20-gallon tank with 10% ammonia that is roughly 1.5 mL. Bottle strength varies and is often unprinted, so add, test in an hour, and top up toward your target.
What kind of ammonia do you use to cycle a tank?
Plain household ammonia with no detergents, scents or colour. Shake the bottle: if it foams it has additives and is not suitable. Clear janitorial-grade ammonia works well.
How long does a fishless cycle take?
Usually two to six weeks. The tank is cycled when it can clear your dose of ammonia and the resulting nitrite to zero within 24 hours. Test daily and keep re-dosing to feed the bacteria.
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