water
Ammonia vs ammonium
A test kit reports one number. Which form is actually in your tank, and does it matter?
Short answer
They are the same nitrogen in two forms, and pH decides the balance. Free ammonia is highly toxic; ammonium is far less so. A test kit reports the two combined, which is why the same reading can be an emergency or a minor problem.
Side by side
| Dimension | Ammonia (NH₃) | Ammonium (NH₄⁺) | Favours |
|---|---|---|---|
| Charge | Uncharged | Positively charged | — |
| Crosses the gill membrane | Readily | Poorly | Ammonium (NH₄⁺) |
| Toxicity | High | Much lower | Ammonium (NH₄⁺) |
| Dominates at | High pH and high temperature | Low pH and low temperature | — |
| Share at pH 6.5, 25 °C | About 0.2 % | About 99.8 % | — |
| Share at pH 8.5, 25 °C | About 15 % | About 85 % | — |
| Preferred plant nitrogen source | — | Yes | Ammonium (NH₄⁺) |
| Reported separately by hobby test kits | No | No | Neither |
Which to choose
Ammonia (NH₃)
Not a choice — but if your pH is above about 7.5, treat any ammonia reading as urgent.
Ammonium (NH₄⁺)
At pH below about 7, the same reading is much less dangerous — though it still means the filter is not keeping up.
One caveat
Never raise pH while ammonia is present. Doing so converts harmless ammonium into toxic ammonia and can kill a tank that was coping.