Free Ammonia Toxicity Calculator - NH3 Level | Fish Tank Calculator

Calculate toxic unionized ammonia (NH3) from total ammonia, pH, and temperature. Know when your ammonia reading is actually dangerous to fish.

Total ammonia in aquarium water exists in two forms: ionized ammonium (NH4+, relatively harmless) and free ammonia (NH3, acutely toxic to gills). The split between them shifts dramatically with pH and temperature, so the same 1 ppm total ammonia reading can be benign at pH 6.5 / 22 degC and lethal at pH 8.2 / 28 degC. This calculator turns your measured total ammonia, pH, and temperature into the actual NH3 fraction in ppm, the most decision-relevant number in any cycling, fish-in emergency, or quarantine scenario.

How It Works

We compute the NH3 mole fraction from the Emerson equation using temperature-dependent pKa values, multiply by your measured total ammonia, and report toxic NH3 ppm alongside the percentage of total that is free ammonia. Status thresholds are conservative: below 0.02 ppm NH3 is safe for most species, 0.02-0.05 ppm requires caution, and above 0.05 ppm requires immediate action (water change, Prime/Amquel detox, or transfer). Cold-water species like trout and goldfish need stricter thresholds than tropicals.

Usage Scenarios

  • Triaging a fish-in cycling crisis at 1.0 ppm total ammonia, pH 7.8, and 26 degC to decide between a 50% change and a switch to a slower fishless protocol.
  • Comparing the actual NH3 risk in a low-pH (pH 6.0) blackwater tank versus a hard-water (pH 8.2) African cichlid tank with the same 0.5 ppm total ammonia reading.
  • Deciding whether to dose a detoxifier such as Prime after a partial bacterial bloom or filter media swap.
  • Quarantine planning: confirming that ammonia accumulation in a bare-bottom hospital tank stays below the species-safe NH3 threshold between daily changes.

How to Use the Ammonia Toxicity Calculator

Test total ammonia (NH3 + NH4⁺) with a standard aquarium test kit (any reagent-based ammonia test kit), then enter that value along with current pH and water temperature. The calculator outputs the fraction that is toxic NH3.

The EPA chronic toxicity threshold is 0.005 ppm NH3. Even with total ammonia at 1 ppm, a tank at pH 8.0 and 28°C can have 0.07 ppm NH3 — far above the chronic danger threshold. This is why pH and temperature context matter.

If NH3 is above 0.005 ppm, perform an immediate partial water change, stop feeding, and identify the ammonia source. Adding a dechlorinator like ammonia detoxifier temporarily detoxifies ammonia while biological filtration catches up.

Frequently Asked Questions

What is the difference between NH3 and NH4?

Total ammonia exists as two forms: toxic unionized ammonia (NH3) and relatively harmless ionized ammonium (NH4⁺). At higher pH and temperature, more ammonia exists as the toxic NH3 form. A reading of 1 ppm total ammonia at pH 8.0 is far more dangerous than at pH 7.0.

What NH3 level is dangerous to fish?

The EPA (1999) chronic toxicity threshold for fish is 0.005 ppm NH3. Above 0.005 ppm causes chronic stress and long-term gill and immune damage. Above 0.05 ppm is acutely dangerous and can kill sensitive fish. Even brief exposure above 0.005 ppm over extended periods can impair gill function.

Why does temperature affect ammonia toxicity?

As temperature rises, the equilibrium shifts toward NH3 (unionized form), increasing toxicity. A tank at 30°C has roughly twice the toxic NH3 fraction as the same tank at 20°C with identical total ammonia and pH.

What should I do if NH3 is dangerous?

Do an immediate 25–50% water change. Stop feeding. Check for dead fish or decaying matter. Add ammonia-neutralizing conditioner (an ammonia detoxifier). Improve filtration. Reducing pH slightly also reduces NH3 toxicity.

How do I test total ammonia?

Use an reagent-based ammonia test kit or digital ammonia meter. Reagent kits measure total ammonia (NH3 + NH4⁺). This calculator then tells you what fraction of that total is the dangerous NH3 form based on your pH and temperature.