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Nobody tells you the exact number. Every guide says dose to 4 ppm, none of them say how many millilitres that is for your tank, with your bottle, this morning. So you end up guessing, or pouring a bit in and testing, and pouring a bit more.
Put your numbers in and it tells you. Tank volume, what your test kit is reading right now, and which ammonia you bought.
Why 4 ppm
Because that is roughly what one axolotl's waste does to a tank. A filter that can clear 4 ppm of ammonia in 24 hours is a filter that can carry the animal. Aim lower and you finish your cycle with a colony too small for the job, then watch ammonia appear a fortnight after your axolotl moves in.
It is also why the topping-up option matters. Once your cycle is running you will rarely be dosing from zero. Tell it what the test kit says now and it works out the difference rather than the full dose.
Knock Out and Dr Tim's are not interchangeable
This catches people out, and it is worth understanding rather than just trusting the buttons.
Knock Out is a household ammonia solution, sold by percentage strength. Dr Tim's is ammonium chloride, and its label is written in mg/L of nitrogen rather than whole ammonia. Those are two different ways of counting the same element, and they differ by about 20%.
The practical upshot is that Dr Tim's is roughly half the strength, so you need about twice the millilitres for the same test kit reading. Use the wrong figure and you will dose half what you meant to, then spend a week wondering why nothing is happening. Switch the button and the maths follows.
If you are using something else entirely, choose the third option and type in its percentage. It must be clear, unscented, and free of surfactants. Cloudy ammonia foams because it contains detergent, and detergent kills the bacteria you are trying to grow.
Measuring it
Use a syringe, not a cap or a spoon. Doses run from about 2 ml for a small tank to over 20 ml for a large one on Dr Tim's, and a millilitre either way is a real difference at these volumes.
Add it, let the filter circulate for an hour, then test. Do not test immediately and do not chase the number with more ammonia before it has mixed. If you overshoot past 8 ppm you will inhibit the bacteria rather than feed them, and the fix is a water change, which is a nuisance.
Next
- The full fishless cycling guide. What to do with the ammonia once it is in, why your pH falls, and how to tell when you are actually finished.
- Watch a cycle run in five minutes. Put your own water numbers in and see how long yours will take, and where it would stall.
- Knock Out Household Ammonia. Clear, 9.5%, and a 500 ml bottle covers a great many cycles.
- API Freshwater Master Test Kit. There is no point calculating a dose you cannot measure the result of.
Wormi Vermi
Fishless cycle · Ammonia dose
Add this much ammonia
4.39
ml
into a 100 litre tank, dosed to 4 ppm using Knock Out ammonia.
Reading
0 → 4 ppm
0
1
2
3
4
Currently reading
3ft ≈ 95L · 4ft ≈ 180L
litres
Ammonia source
% ammonia solution
What to buy
Knock Out Household Ammonia is what most UK axolotl keepers reach for. 9.5% strength, clear, unscented, ready straight from the bottle. Dr Tim's is the aquarium-specific alternative and works just as well, but it is not interchangeable: it is roughly half the strength, so you need about twice the millilitres, and it is dosed in nitrogen rather than whole ammonia. Switch the button and the maths follows. Anything else off a supermarket shelf could foam, could smell, could kill your cycle before it starts.
Why don't the numbers match other calculators?
Two conventions measure ammonia in this industry. NH3 counts the whole molecule. NH3-N counts only the nitrogen inside it. They differ by roughly 20%. Each button here is calibrated to its own manufacturer's specification, so your dose matches what's printed on the bottle in your hand. Cross-check against a generic calculator using the other convention and you'll see the gap. Your test kit reading is what actually counts.