Think of ammonia as the first dissolved nitrogen waste from the fish. The fish eats protein in the feed. Some of that nitrogen becomes new fish (growth). Some is converted in metabolism to ammonia and leaves the body, mostly across the gills. What we measure in the water is called TAN - total ammonia nitrogen. TAN is the sum of two forms: NH₄⁺ (ammonium), which is relatively harmless at ordinary levels, and NH₃ (unionised ammonia), which is highly toxic even at low concentrations.
Why TAN is one of the most important parameters
In most intensive systems ammonia is not just a water-quality number. It shows whether biological treatment is keeping up with fish metabolism. Every gram of feed eventually becomes fish biomass, solid waste or dissolved waste. The most important dissolved nitrogen waste is ammonia.
Unlike many other problems, TAN can rise quickly after feeding, a biofilter disturbance, equipment failure, a power cut or a sudden biomass increase. Fish produce ammonia continuously - there is no practical way to switch the source off without cutting feed or removing fish. Rising TAN is therefore often an early warning, before fish show clear stress.
A steadily rising TAN level may indicate:
- Biofilter overload, insufficient capacity, or reduced nitrification.
- Low alkalinity or low DO in the biofilter.
- Excessive feeding or inadequate water exchange.
- Recent disinfection or medication that hit nitrifiers.
Where does TAN come from?
Start with the feed. The fish eats protein. In the body, protein is broken down into amino acids. Some are used to build growth. Some are used as energy. When amino acids are burned, the nitrogen is left over - it is converted to ammonia (TAN). Some of this TAN stays briefly in the fish, but most is excreted into the water, mainly across the gills. So it is not "unused protein dumped as-is". It is protein the fish actually ate and metabolised - and then released as TAN.
That is why TAN follows the feed: more feed → more metabolised protein → more TAN in the water. For salmonids in good growth a common rule of thumb is about 25–45 g TAN-N per kg dry feed. The figure rises if the feed is protein-rich, if the fish grow poorly (bad FCR), or if more protein goes to energy than to growth. Peak feed means peak TAN. See the mass-flow sketch under dissolved oxygen.
Two forms: the harmless and the toxic
TAN in the water is always a mixture. Think of a seesaw: NH₄⁺ ⇌ NH₃ + H⁺. On one side sits the relatively harmless ammonium ion. On the other the toxic gas form NH₃. Raise pH (fewer hydrogen ions) and the board tips toward NH₃. Raise temperature and it tips the same way, more moderately.
Three things to keep in mind: TAN says how much ammonia is there in total. pH decides how large a share becomes toxic NH₃. Temperature raises that toxic share further. Always read the three together. In freshwater we often use Emerson et al. (1975) - that is what the calculator below does. In seawater the equilibrium is almost the same, but not identical. See pH and temperature.
Pourbaix diagram (Eh–pH)
A Pourbaix diagram shows which chemical form is thermodynamically most stable at a given pH and redox potential. Horizontal is pH. The left vertical axis is Eh in volts vs SHE (the lab reference). The right axis is ORP in mV vs Ag/AgCl — what most farm probes display. Roughly: Eh (V) ≈ ORP (V) + 0.20. High Eh/ORP = oxidising (oxygen present). Low = reducing. Dashed lines are water's own window: above the upper line water would want to make O₂, below the lower line H₂.
For nitrogen, the vertical line at pKa ≈ 9.25 is the same seesaw as the calculator: NH₄⁺ to the left, NH₃ to the right. Up and down is oxidation state: NO₃⁻ at the top (oxic), then a thin NO₂⁻ band, then NH₄⁺/NH₃ at the bottom (reducing). N₂ is not drawn — it would cover almost the whole box, but N≡N is so inert that it is not the form we steer day to day in the tank.
In an oxygenated tank nitrogen "wants" to sit as nitrate. The path TAN → NO₂⁻ → NO₃⁻ is slow without bacteria. The biofilter is that path. The black point is a typical well aerated tank (oxic) — in the nitrate field, left of NH₃. The fish can still have TAN if the filter does not keep up. A nitrite spike is getting stuck in the thin middle band.
Metastable farm diagram, 25 °C. N₂ omitted: thermodynamically huge but kinetically inert — in practice we see NH₄⁺/NH₃, NO₂⁻ and NO₃⁻. Left: Eh (SHE). Right: ORP (mV, Ag/AgCl). Vertical line: pKa ≈ 9.25. The biofilter is the kinetics between fields.
© 2026 DABCE ABCalculator: TAN → NH₃ (unionised)
© 2026 DABCE AB
Interpreting the calculator
The calculator takes your TAN, pH and temperature and answers: how much is toxic NH₃ right now? That is a more useful question than only "how much ammonia is in the sample?". The same 1 mg/L TAN can be almost harmless in cool, near-neutral water and serious in warm, alkaline water. A common mistake is to look only at the TAN number.
| TAN | pH | Temp. | Approx. NH₃-N | Approximate risk |
|---|---|---|---|---|
| 1.0 mg/L | 7.0 | 14 °C | ≈ 0.003 mg/L | Usually low |
| 1.0 mg/L | 8.0 | 18 °C | ≈ 0.03 mg/L | Significantly higher |
| 1.0 mg/L | 8.5 | 25 °C | ≈ 0.15 mg/L | Potentially dangerous |
Same TAN = 1.0 mg/L. NH₃-N from Emerson (freshwater), rounded. Try the same values in the calculator.
Orientation - not a limit value
| NH₃-N (unionised, mg/L) | General reading |
|---|---|
| < 0.012 | Often a comfortable salmonid / hatchery target |
| 0.012–0.05 | Raised chronic risk - cut TAN or pH, check feed |
| 0.05–0.20 | Clear risk, especially with long exposure |
| > 0.20 | Potentially acute depending on species and time |
Common orientation bands in freshwater salmonid literature. Warm-water species often tolerate more. Not a regulatory value — species, life stage and exposure time govern. See also the combined rainbow trout diagram under nitrite (TAN, NH₃, NO₂, NO₃).
What if NH₃ is too high?
When unionised ammonia approaches problematic levels, operators generally have a few options:
- Cut production: temporarily less feed, fewer meals or lower biomass - that lowers TAN at the source.
- Improve biological conversion: DO and alkalinity/pH in the biofilter, remove sludge, confirm nitrification capacity. A healthy filter takes TAN → nitrite → nitrate.
- Dilute: more make-up, higher flow-through. Lowers TAN quickly but costs water and often energy.
- Reduce the NH₃ fraction: in some cases a careful pH drop cuts toxicity even if TAN is unchanged. Many intensive RAS plants run a little below natural-water pH - but pH must be balanced against the biofilter and alkalinity. See pH.
TAN as a process-control parameter
In modern RAS, TAN is increasingly treated as a process-control parameter, not only as water quality. Oxygen shows whether fish can breathe; TAN shows whether treatment is keeping pace with feed and growth. Successful farms rarely look at TAN alone. They follow the whole nitrogen chain Feed → TAN → nitrite → nitrate together with pH → alkalinity → CO₂ → dissolved oxygen. Together that is a clearer picture than any single measurement.
In RAS and many semi-flow systems, nitrifiers convert TAN → NO₂⁻ → NO₃⁻. That lowers TAN but consumes oxygen and alkalinity and can give a nitrite spike at start-up or if the biofilter is knocked out. Nitrate accumulates until you exchange water or denitrify. See nitrite, nitrate, pH and dissolved oxygen.
Differences between systems
- Ponds: TAN is diluted and partly taken up by algae; daily pH drives NH₃ risk more than average TAN.
- Flow-through / raceway: flow dilutes TAN - too little water means both oxygen and ammonia risk.
- Net pen: large volume, but local TAN in dense fish and weak current; seawater pH (~8) means a higher NH₃ share.
- RAS: TAN is a process parameter. Biofilter, peak feed and pH/CO₂ matter more than dilution.
Measurement
Spot checks (kit or lab) are common. Online TAN exists but is costlier and needs care. More important than the instrument: measure where the fish live, together with pH and temperature, and after the feed peak - not only in the morning in a sump. Compare with nitrite if the biofilter is young or recently disturbed.
Signs of ammonia stress
Reduced appetite, increased gill ventilation, darker colour, surface behaviour, poorer growth and in the worst case gill damage and death. The picture resembles CO₂, low DO and nitrite - measure TAN, pH, T, NO₂⁻ and DO before changing everything at once.
Practical rule of thumb
Fish produce TAN continuously. Biofilters remove TAN continuously. The operator's job is to keep removal ahead of production. Never interpret TAN without also looking at pH and temperature. The fish care about NH₃, not TAN.
Sources/overview (selection): Emerson et al. (1975) for the freshwater equilibrium; Timmons & Ebeling, Recirculating Aquaculture; Boyd & Tucker; established RAS practice. The calculator is freshwater - seawater shifts pKa slightly. Ranges are orientation, not permit values.