Ammonia in water: NH₃ or NH₄⁺ - why the form decides toxicity
A result from a laboratory or probe says: ammonium nitrogen (N-NH₄) = 1.2 mg/l. Is that a lot or a little? The answer is: it depends on pH and temperature - because the same figure can mean water that is safe for fish or water in which organisms begin to suffer. The key is that ammonium nitrogen is the sum of two forms, and only one of them is truly dangerous.
TAN, the sum of two forms
The laboratory result TAN (Total Ammonia Nitrogen) is the sum of:
- the ammonium ion (NH₄⁺) - a form that is in practice of low toxicity,
- un-ionised ammonia (NH₃) - a highly toxic form, because as a neutral molecule it freely passes through the gill membranes.
These two forms remain in chemical equilibrium with each other: NH₄⁺ ⇌ NH₃ + H⁺. Which way this equilibrium is shifted is decided by pH (mainly) and temperature (additionally).
Why pH matters so much
At low pH (acidic or neutral water) the equilibrium is strongly shifted towards NH₄⁺ - even at high TAN the share of toxic NH₃ is negligible. But every rise in pH by one unit shifts the NH₃/NH₄⁺ ratio about tenfold. At pH 9, from the same TAN there is an order of magnitude more un-ionised ammonia than at pH 7.
That is why two ponds with identical TAN can carry quite different risk - if one has pH 7 and the other 8.5 (which in eutrophic water bodies with intensive photosynthesis happens entirely naturally during the day).
The role of temperature
Temperature acts in the same direction as pH, though more weakly: warmer water shifts the equilibrium towards NH₃. In practice this means that in summer, with higher temperature and raised pH (algal photosynthesis), the risk of ammonia toxicity accumulates - and it is precisely then, in summer, in warm and fertile water bodies, that fish kills most often occur, explained as “lack of oxygen”, though ammonia can be a co-cause.
Effects on aquatic organisms
Un-ionised ammonia damages the gill epithelium, impairs gas exchange and weakens immunity - even at concentrations that do not kill immediately. The most commonly cited indicative thresholds for NH₃-N in farmed and natural waters:
- below ~0.02 mg/l NH₃-N - usually safe for sensitive species,
- ~0.02-0.05 mg/l - a warning range, harmful to growth and condition under prolonged exposure,
- above ~0.1-0.2 mg/l - a risk of acute stress, lethal for some species.
These are indicative bands, not a single universal legal limit - sensitivity differs between species (e.g. salmonids are much more sensitive than cyprinids), and the binding standards depend on the water’s intended use. Treat them as a signal of when it is worth taking a closer look, not as a verdict.
When the risk really rises
- Aquaculture ponds - high stocking, intensive feeding → high TAN from fish metabolism and feed breakdown,
- Eutrophic ponds and lakes - algal photosynthesis raises pH during the day, sometimes above 9,
- After water exchange or storms - sudden changes in pH/temperature can abruptly shift the NH₃/NH₄⁺ equilibrium even at constant TAN.
In practice
A TAN result alone says little without pH and temperature measured at the same moment - ammonia responds to pH changes within minutes, so a sample from a different time of day can give a misleading picture. On converting TAN units (e.g. NH₄ ↔ N-NH₄) see Converting concentration units.
🧮 Tool: Ammonia toxicity calculator (NH₃/NH₄⁺)
- enter TAN, pH and temperature, and we will compute the concentration of toxic NH₃ and compare it with the indicative thresholds.
How to make it easier
For an ammonia assessment to make sense, TAN, pH and temperature must be measured together and recorded together - and ideally as a series, because a single reading from a windless afternoon (high pH) can look quite different from a reading at dawn.
In LimnoLog you record all these indicators in a single measurement session - right in the field, also offline - and you see them together on a chart at once. You will easily notice whether the daytime rise in pH really correlates with TAN and whether you are approaching the risk zone.
← Knowledge baseEarly users get free access for now and keep their features for good. If you run a farm or monitor a water body - take a look and try it.