Ammonia is the number one killer of fish in new, crowded, or overfed tanks and ponds. But the reading on your liquid test kit only tells half the story. The amount that actually burns gills depends on your pH and temperature — which is why the same reading can be harmless in the morning and deadly by the afternoon.
Total ammonia vs. toxic ammonia (NH₃)
Your test kit measures total ammonia nitrogen (TAN), which is a mix of two forms:
- Ammonium (NH₄⁺) — largely non-toxic.
- Unionized ammonia (NH₃) — the toxic form that damages gills.
Only the NH₃ share matters for safety. As pH rises, the toxic NH₃ fraction increases roughly tenfold for every one unit of pH, and it also rises with temperature. So a tank at pH 8.4 and 28 °C is far more dangerous at the same test reading than one at pH 6.8 and 22 °C.
Calculate your toxic NH₃ (free) →
Safe ammonia level chart (NH₃)
- Below 0.02 mg/L — Safe for long-term exposure.
- 0.02 – 0.05 mg/L — Caution: fish are stressed and eat less.
- 0.05 – 0.2 mg/L — Danger: gill damage is likely.
- Above 0.2 mg/L — Severe: often lethal, especially for fry and fingerlings.
These thresholds are general guidelines used widely in aquaculture; sensitive species and young fish react at the lower end of each band.
What to do when ammonia is high
- Change water — 20–30% with dechlorinated, temperature-matched water. Repeat if needed.
- Stop or cut feeding for a day or two. Uneaten food is a major ammonia source.
- Add aeration — stressed gills need more oxygen.
- Do NOT raise pH. Higher pH turns more of your ammonia into the toxic NH₃ form. Fix the ammonia first.
How to prevent ammonia spikes
- Cycle a new tank or pond before adding many fish, so beneficial bacteria can process ammonia.
- Avoid overstocking — more fish means more waste. (See our stocking density calculator.)
- Feed only what fish finish in a few minutes.
- Test regularly, especially in warm weather and after adding fish.
Frequently asked questions
Is 0.25 ppm ammonia safe?
It depends entirely on pH and temperature. At low pH and cool water it can be nearly harmless; at high pH and warm water the same 0.25 ppm total can be dangerous. Convert it to NH₃ before deciding.
Does a planted tank need less worry?
Live plants absorb some ammonia, which helps — but they do not remove the risk during a spike. Still test and act on high readings.
My fish are gasping at the surface — is it ammonia?
It can be ammonia-damaged gills, low oxygen, or both. Add aeration and do a water change immediately, then test.