Why pH and temperature matter so much
Ammonia exists in water in two forms: ammonium (NH₄⁺), which is relatively harmless, and free or unionized ammonia (NH₃), which is toxic because it passes easily through the gills. Most test kits measure both together.
The toxic share depends on pH and temperature. Every one-unit rise in pH increases toxic ammonia roughly tenfold. That is why a pond can be safe at dawn (pH 7) and dangerous in the afternoon (pH 8.5–9), especially green ponds with heavy algae, where photosynthesis drives the pH up during the day.
Thresholds used by this calculator
| Toxic ammonia (NH₃) | Status | Meaning |
|---|---|---|
| < 0.02 mg/L | ✅ Safe | Long-term safe limit widely used in aquaculture guidelines |
| 0.02–0.05 mg/L | 🟡 Caution | Stress, reduced appetite and growth |
| 0.05–0.2 mg/L | 🟠 Danger | Gill damage, higher disease risk |
| > 0.2 mg/L | 🔴 Severe | Can be lethal for many species, especially fry |
Tolerance varies by species: catfish are relatively hardy, while fry, trout and many ornamental fish are more sensitive. These are general reference values, not exact limits for every species.
What to do when ammonia is high
- Reduce or stop feeding for 1–2 days. Uneaten feed and waste are the main ammonia sources.
- Change 20–30% of the water gradually, with clean water at a similar temperature.
- Increase aeration.
- Remove sludge and waste from the bottom (siphon). In aquariums, check that the filter is not overloaded and has not been cleaned too aggressively.
- Do not raise the pH (lime, buffers) while ammonia is high, because it increases the toxic form.
- Test again in the afternoon, when pH peaks, not only in the morning.
Reading your test kit
Check the label. Some kits report NH₃/NH₄⁺ (total ammonia), others report NH₃-N or TAN (total ammonia expressed as nitrogen). Choose the matching option above. The calculator converts automatically (1 mg/L as N = 1.216 mg/L as NH₃).
Formula: NH₃ fraction = 1 / (10(pKa − pH) + 1), with pKa = 0.09018 + 2729.92 / (T °C + 273.15). Source: Emerson et al. (1975), J. Fish. Res. Board Can. 32: 2379–2383. Valid for fresh water; in seawater the toxic fraction is slightly lower.