Free Ammonia Calculator.
Free Ammonia Calculator
How much of your ammonia reading is the kind that hurts fish.
Why the test number is not the answer
An ammonia test or probe reports total ammonia: two forms added together. Most of it is ammonium (NH4+), which fish tolerate fairly well. A small share is free ammonia (NH3), which passes through gills and does the damage. pH and temperature decide how big that share is, and the share climbs fast. At 77°F (25°C), about 0.6% of total ammonia is free at pH 7, about 5% at pH 8, and about a third at pH 9. Warmer water pushes every one of those numbers up.
So the same 1 mg/L reading can be harmless in an aquaponic system held near pH 7 and serious in a pond that swings to pH 9 on a sunny afternoon. That is why a pH probe belongs next to any ammonia probe: without it, the ammonia number cannot tell you how worried to be.
Reading the result
The status line uses general guide values for free ammonia (as NH3) in warm-water food fish:
| Free NH3 (mg/L) | What it usually means |
|---|---|
| Under 0.02 | Low. Fine for long-term exposure in most systems. |
| 0.02 to 0.05 | Watch it. Growth and health can suffer with weeks of exposure. |
| 0.05 to 0.2 | Stress likely. Feed less, add water exchange or aeration, find the cause. |
| Over 0.2 | Dangerous. Act now; many species die in the range above this. |
Species differ. Trout, fry and shrimp larvae are more sensitive than adult tilapia or catfish, so treat these as a starting point and check the guidance for what you raise.
Aquaponics, ponds and salt water
- Aquaponics holds pH near 6.8 to 7.0 for the plants, which keeps the free share small. There, a rising total ammonia is mostly a sign that the biofilter is behind (a new system still cycling, or a filter that crashed), and the fish usually feel nitrite and low oxygen first.
- Ponds swing in pH through the day as algae take up carbon dioxide in sunlight. Run the calculator at the afternoon pH, not the morning one.
- Salt and brackish water hold slightly less free ammonia at the same pH and temperature. This calculator uses the freshwater equation, so in salt water it slightly overstates the risk, which is the safe direction to be wrong.
The math
The share of total ammonia that is free NH3 follows from the acid dissociation constant of ammonium, which depends on temperature (Emerson and others, 1975, freshwater):
pKa = 0.09018 + 2729.92 / T free share = 1 / (1 + 10^(pKa − pH)) T is water temperature in kelvin (°C + 273.15). Free NH3 (as NH3) = total ammonia as N × free share × 1.216
The 1.216 converts nitrogen to the mass of the NH3 molecule (17.03 / 14.01). If your test reports total ammonia as NH3 or as NH4+, the calculator converts it to nitrogen first.
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