TOOL 08
Ammonia Toxicity
Calculator.
Your ammonia test kit reports total ammonia — the harmless ammonium ion plus the toxic free gas. This tool splits them apart, because pH and temperature decide how much is actually dangerous.
Free ammonia fraction = 1 ÷ (1 + 10(pKa − pH)), where pKa = 0.09018 + 2729.92 ÷ (T + 273.15). Toxic NH3 is read against the free-ammonia column of your test kit, not the total.
ABOUT THIS TOOL
Ammonia Toxicity
Calculator.
Most ammonia test kits measure total ammonia nitrogen (TAN), which is the sum of two forms: ionised ammonium (NH4+), which is relatively harmless, and un-ionised free ammonia (NH3), which is highly toxic even at tiny concentrations. The balance between them depends on pH and temperature, so the same test reading can mean very different things in two tanks.
In acidic water, almost all ammonia exists as the safe ammonium ion. In alkaline water, a much larger share becomes free ammonia. Rising temperature pushes the balance further toward the toxic form. This is why a pH of 8.0 at 28°C can be far more dangerous than the same ammonia reading at pH 6.5 and 24°C.
The calculator converts your total ammonia reading into a free-ammonia concentration using the standard pKa relationship. You can then weigh it against the commonly cited toxicity thresholds: below 0.02 ppm is generally safe, 0.02–0.05 ppm warrants a water change, and above 0.05 ppm demands immediate action.
HOW TO USE
Using the
Calculator.
- Test total ammoniaUse a liquid ammonia test kit and record the result in ppm (mg/L). This is your total ammonia reading.
- Measure your pHTake a fresh pH reading from the same water sample at the same time, so temperature and chemistry match.
- Record the temperatureNote the aquarium temperature in Celsius. Warmer water shifts more ammonia into the toxic form.
- Enter all three valuesType the readings into the fields above. The output updates instantly as you type.
- Read the free ammoniaThe primary result is un-ionised ammonia (NH3) in ppm. This is the number that actually harms fish.
- Act on the risk levelFollow the safety readout: perform a water change for anything in the caution band, and act immediately above the danger threshold.
FAQ
Ammonia
Questions.
What is a safe level of free ammonia?
Most references treat un-ionised ammonia below 0.02 ppm as safe for long-term health. From 0.02 to 0.05 ppm you should perform a water change and find the source. Anything above 0.05 ppm is dangerous and requires immediate dilution.
Why does pH matter so much?
The ammonium/ammonia balance moves sharply with pH. At pH 6.0, well over 99% of total ammonia is the harmless ion. At pH 8.5, a large fraction becomes toxic free ammonia. A one-unit pH rise can multiply the toxic fraction several times over.
Does temperature really change the risk?
Yes. Higher temperatures shift the balance toward free ammonia and also increase the fish metabolism that makes ammonia more damaging. A hot, alkaline tank is the worst combination for a given total ammonia reading.
My test reads zero — is the calculator pointless?
Not at all. It is most useful right after a spike, when your kit shows a small positive reading. That is exactly when knowing the free-ammonia share tells you whether to relax or act immediately.
What should I do if free ammonia is high?
Perform a large, temperature-matched water change, stop feeding, ensure the filter is running, and investigate the cause — overstocking, a dead fish, overfeeding, or a crashed biofilter. Add a bacterial starter if the cycle has stalled.
Does a conditioner make ammonia safe?
Some conditioners detoxify ammonia temporarily by binding it, but the effect is finite and does not replace water changes. Use them as a stopgap while you dilute the tank, not as a permanent fix.