Fishless cycling: how much ammonia to add and what to test
AquaLog guide · Published
In short: Check the strength on the label, estimate a starting dose (2 ppm means 2 mg of ammonia in every litre), add about half, test, and build up to about 2 ppm. Top back up whenever ammonia falls below 1 ppm, test ammonia and nitrite every two or three days, and check pH and KH weekly. The cycle is done when ammonia and nitrite both read 0 ppm within 24 hours of a 2 ppm dose, usually after 4–8 weeks.
Before you start
This guide builds on how to cycle an aquarium, which explains the nitrogen cycle, the equipment and the basic fishless method. Here are the parts it leaves out: how much ammonia to add, a testing log to follow, and what to do when the cycle stalls.
On top of the kit listed there, you need a KH test, a syringe or measuring spoon kept only for ammonia, and your tank's real water volume. Gravel, rock and the gap below the rim take up space, so work it out with the aquarium volume calculator.
How much ammonia should you add?
Products differ in strength, so never copy someone else's millilitre figure. Check the label, estimate a starting amount, and let your test kit have the final say. If the product is sold for fishless cycling, follow its label.
The target is about 2 ppm, which is 2 mg of ammonia in every litre of water. If the label gives the ammonia content as a percentage, each 1% is about 10 mg per ml. That gives a starting estimate: ml for 2 ppm ≈ 2 × litres ÷ (label percentage × 10). For example, if the label says 5% and the tank holds 100 litres (26 US gal): 2 × 100 ÷ 50 = 4 ml. If the label gives no strength, skip the estimate and start with a few drops.
- Wear gloves, open a window, and keep the ammonia in its original, labelled bottle, away from children. Never mix ammonia with bleach or other cleaners: together they release toxic chloramine gases.
- Add about half your estimate, measured, and write down how much.
- Wait until the filter has mixed it through the whole tank, then test.
- Scale up. Ammonia rises in proportion to the amount you add, so if 2 ml gave about 1 ppm, about 4 ml in total gives about 2 ppm. Add the rest in one or two steps, testing as you go.
- Write down the total that gave about 2 ppm. That is your full dose for this tank.
- If you overshoot, a water change brings it down: changing half the water takes 4 ppm to 2 ppm. The water change calculator works out other amounts. Enter your treated tap water's ammonia reading too, as chloramine-treated water can carry some.
Most test kits read total ammonia in mg/L, which is the same as ppm. Dose against your test, not the bottle.
How much do you add when topping up?
Top up whenever ammonia falls below 1 ppm. You only need enough to get back to about 2 ppm: top-up ml ≈ full dose × (2 − current reading) ÷ 2. At 0.5 ppm that is three-quarters of your full dose; at 0 ppm it is all of it. If you have just changed some water, test again before you top up, because the change lowers ammonia too.
What should your testing log look like?
Test at the same time of day, with the same kit and the same method, so each reading compares fairly with the last. Write down every result and every ml of ammonia you add.
| When | Test | What to do |
|---|---|---|
| Before you start | Treated tap water: ammonia, pH and KH | If treated tap water shows ammonia, your supply may use chloramine. Knowing that stops it confusing later readings. |
| Day 1 | Ammonia, once it has mixed in | Dose to about 2 ppm and record the amount. |
| Day 2 | Ammonia | Confirm the dose held. |
| Every 2–3 days | Ammonia and nitrite | Top up to about 2 ppm whenever ammonia is below 1 ppm. |
| Once a week | pH, KH, nitrate and temperature | If pH is sliding towards 6.5 or KH is near zero, see the stall checks below. Rising nitrate shows the second stage is working. |
| Once nitrite is falling | Ammonia and nitrite, 24 hours after a full dose | Both at 0 ppm, with nitrate clearly above zero, means the cycle is done. Repeat on a second day to be sure. |
| Stocking day | Nitrate | Do a large water change to bring nitrate down (the water change calculator sizes it), then add your first few fish. |
The stages themselves are described in the cycling guide. Most tanks finish in 4–8 weeks, and the nitrite stage is often the slowest.
If you can't add fish straight away, keep dosing ammonia as before. Without it the bacteria die back, and the tank loses capacity it took weeks to build.
Plotting ammonia and nitrite each time you test makes a stall obvious. AquaLog keeps every reading on a chart per tank. Log your cycle.
Why has my cycle stalled?
If nothing has changed for a week or more, test pH and KH before anything else. These are the usual causes:
- pH has dropped. Nitrifying bacteria release acid as they work and use up carbonate. In soft water the pH can slide below about 6.5, where the bacteria slow down sharply. A partial water change helps if your tap water's pH and KH are higher than the tank's.
- KH has run out. Below about 20 mg/L (roughly 1 dKH), the common nitrifying bacteria stop working effectively. Raise KH a little, for example with a small, measured amount of baking soda (sodium bicarbonate) dissolved in tank water, then re-test. Add it in steps, the same way you dose ammonia. See KH vs GH in an aquarium.
- The water is cold. Check the heater against a thermometer; the bacteria grow faster in warm water. If your fish need cooler water, such as fancy goldfish at 16–23 °C (61–73 °F), don't cycle far above their range. One aquaculture fact sheet advises cycling no more than 2–3 °C (4–5 °F) above the temperature you will keep, so the bacteria don't die back when you cool the tank.
- Nitrite is stuck very high. If nitrite climbs to the top of your test's scale and stops moving, a partial water change can help.
- Untreated top-ups. Topping up evaporation straight from the tap adds chlorine or chloramine, which kill the bacteria. Treat every top-up.
- Ammonia ran out. Bacteria starve when ammonia reads 0 for days. Top up whenever it falls below 1 ppm.
- Too little oxygen. The bacteria need oxygenated water. Keep the filter running and the surface moving.
- Chemicals. Medicines, especially antibiotics, and cleaning products can damage the bacteria. Keep them out of a cycling tank.
- An old test kit. Expired reagents give false readings. Check the date on the bottles.
If it still won't move, a little filter media from an established, healthy aquarium is the most effective way to add bacteria. Keep dosing and testing as before.
What comes next?
Add a few hardy fish first, such as those on our beginner fish list, then build up slowly as described in how to set up your first freshwater aquarium. Start the maintenance checklist from week one.