Ammonium / Ammonia (NH₄ / NH₃)
The first step of the nitrogen cycle and the acute fish-killer. Should read zero in any cycled tank.
What it is
Ammonium is the nitrogen waste fish, shrimp, and decaying plant matter excrete directly into the water. In a cycled tank, a colony of nitrifying bacteria converts it to nitrite within hours; in a new, uncycled tank, it accumulates.
A pH-dependent share of ammonium exists as the uncharged ammonia (NH₃), which is what actually crosses fish gill membranes and does the damage. Below pH 7 almost all of it is the relatively harmless NH₄⁺; above pH 8 it shifts sharply toward toxic NH₃. Test kits usually report total NH₄/NH₃; the same number is much more dangerous in a high-pH tank than a low-pH one.
Why it matters
Ammonia is acutely toxic at concentrations below the resolution of most test kits — fish in a high-pH tank can die at "0.25 mg/L" if a quarter of that is NH₃. It's the parameter that kills new tanks before the cycle finishes.
How to test
JBL NH₄, Tetra NH₃/NH₄, or Salifert ammonia kits. Test daily during cycling, after a deep clean, after introducing new livestock, or any time the tank smells off. In an established tank, a monthly check is enough.
What high and low look like
Zero is the only acceptable reading. Above 0.25 mg/L total in a high-pH tank is already lethal for sensitive species; above 0.5 mg/L kills most freshwater fish within a day.
How to fix
50% water change immediately. Add an ammonia-binding conditioner (Seachem Prime, JBL Biotopol C) to neutralise residual ammonia for the next 48 hours. Stop feeding for two days. If readings persist, your biological filter is not yet cycled — wait it out, supplement with bottled bacteria, and don't add more livestock.
