Fishless cycling: how much ammonia to dose

Updated on August 25, 2026 · ~6 min read

In a fishless cycle you grow the bacterial colony by dosing pure ammonia instead of using fish as a waste source. Bring ammonia to 1-2 ppm, top it up as it falls, and call the cycle finished when the tank clears 2 ppm of ammonia in 24 hours leaving zero nitrite.

Growing the bacteria that purify an aquarium requires ammonia. The traditional way to supply it is to put a few "hardy" fish in and let them produce it: that works, but those fish spend three to six weeks in water that damages them, and some do not reach the end.

Fishless cycling does the same job without animals: you dose the ammonia from a bottle. It takes slightly longer to understand and rather less time to carry out, you control exactly how much food the bacteria get, and at the end you have a tank rated for a known load instead of hoping it copes.

What you need

• Pure aqueous ammonia, with no surfactants, no fragrance, no colouring. In practice: technical-grade ammonia, or a product made for aquariums. The label must state the concentration (often 5%, 10% or 24-25%): without that number the dose cannot be calculated. If it foams when shaken, it contains surfactants — do not use it.
• Liquid ammonia and nitrite test kits. Test strips lack the sensitivity needed here.
• A nitrate test, for the final confirmation.
• A graduated syringe, 1 or 5 ml: the doses involved are small.
• A dechlorinator for tap water, and the filter running from day one.

You do not need fish, you do not need plants (they help, but by absorbing ammonia they make the readings harder to interpret), and you do not need light: the tank can stay dark for the whole cycle, which also keeps algae away.

The dose calculation

The arithmetic is simpler than it looks. One ppm is one milligram per litre, so:

mg of ammonia needed  =  target ppm × litres in the tank
mg of ammonia per ml  =  (concentration % / 100) × 900
ml to dose            =  mg needed / mg per ml

The 900 is the mass of one millilitre of solution in milligrams (density ~0.9 g/ml). A worked example, with a 10% solution in a 100 L (26 gal) tank taken to 2 ppm:

mg needed  = 2 × 100      = 200 mg
mg per ml  = 0.10 × 900   =  90 mg/ml
ml to dose = 200 / 90     ≈ 2.2 ml

Tank volume	5% solution	10% solution	25% solution
60 L (16 gal)	~2.7 ml	~1.3 ml	~0.5 ml
100 L (26 gal)	~4.4 ml	~2.2 ml	~0.9 ml
200 L (53 gal)	~8.9 ml	~4.4 ml	~1.8 ml
400 L (106 gal)	~17.8 ml	~8.9 ml	~3.6 ml

The doses in the table take the tank to 2 ppm starting from zero. Treat them as indicative: the concentration on the label is never exact and ammonia loses strength over time. Dose slightly less than calculated, test, and adjust.

Step by step

1. Set up and run. Substrate, decor, treated water, filter and heater on. Temperature matters: 25-28 °C (77-82 °F) is the range where nitrifying bacteria multiply fastest, and keeping it high through the cycle saves days.

2. Dose the first ammonia, to 1-2 ppm. Then test, to confirm you are where you think you are: this is the moment you find out whether the arithmetic held.

3. Test ammonia and nitrite every two days. For a week or two nothing happens. That is normal: the bacteria are there, there are very few of them, and they are multiplying.

4. When ammonia starts to fall, top it back up. The first group is working. It must keep being fed: every time ammonia drops below about 1 ppm, re-dose to 2. Stop feeding it and the young colony starves.

5. Wait for nitrite, then wait for it to fall. It will appear, and it will climb a long way — often off the scale. This is the longest phase: the second group of bacteria grows more slowly than the first and can take two or three weeks. There is nothing to do but keep dosing ammonia and keep testing.

6. The final test. The cycle is complete when, dosed to 2 ppm of ammonia, after 24 hours you read 0 ammonia and 0 nitrite, with nitrate risen. This is the test that counts: it shows not merely that the bacteria exist, but that there are enough of them to clear a real load in a day.

7. A big water change. At the end of the cycle nitrate is very high — it is the product of weeks of dosed ammonia — and must come back below 25 ppm in freshwater, or 1-10 ppm in a reef. That takes a large change, often 70-90%, sometimes two on consecutive days. It does not harm the bacteria: they live in the media and substrate, not in the water.

8. Add fish within a day or two. After the water change the colony has no ammonia left to eat. Leave the tank empty for a week and part of it dies off, undoing progress. A fishless cycle ends by stocking promptly — and it is the only method that allows several fish at once, because the colony is already rated for 2 ppm a day.

How long it takes

Three to six weeks with the temperature high and regular dosing. With some filter media from a healthy established tank it can drop to one or two weeks: again, the most effective shortcut there is, because it does not add bacteria, it transplants a grown colony.

If nitrite is still high after eight weeks, the suspects are the usual ones: chlorinated water at every top-off, temperature too low, ammonia dosed too high (above 5 ppm), too little oxygen, or an expired nitrite test — nitrite reagents are among the first to go bad.

Typical mistakes

• Ammonia with surfactants. It foams, it stays in the tank and it is not easy to remove. Shake the bottle before buying it.
• Dosing once and waiting. Without top-ups the colony grows only as far as the first dose supports, then stops.
• Changing water during the cycle. That takes food away from the bacteria. The water change belongs at the end, not in the middle — the opposite of what you would do with fish in the tank.
• Trusting the ammonia test alone. Ammonia at zero with nitrite never seen means the cycle has not started, not that it has finished.
• Dumping the water and restarting "to do it properly". The bacteria live in the media and the substrate: emptying the tank halfway through throws away weeks of work.

How Gurglee does it

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