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 — and it should read 0 mg/L in any cycled tank. Above 0.1 mg/L is a warning that biological filtration isn't keeping up; above 0.25 mg/L is critical, and in a high-pH tank sensitive species start dying there. In a cycled tank a colony of nitrifying bacteria converts ammonium to nitrite within hours; in a new, uncycled tank it accumulates with nothing to check it.
Ammonia vs ammonium — why pH decides how dangerous it is
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 ionised form, NH₄⁺; above pH 8 it shifts sharply toward toxic NH₃. Test kits report total NH₄/NH₃ combined, so the identical number is far more dangerous in a hard, alkaline tank than a soft, acidic one:
| Tank pH | Share as toxic NH₃ (at 25 °C) |
|---|---|
| 6.5 | ~0.2% |
| 7.0 | ~0.6% |
| 7.5 | ~1.8% |
| 8.0 | ~5% |
| 8.5 | ~15% |
Warmer water pushes every figure higher. This is why a reading of 0.25 mg/L is a shrug in a pH-6.5 blackwater tank and an emergency in a pH-8 rift-lake setup — the pH parameter page covers the buffering side of the same coin.
Why it matters
Ammonia is acutely toxic at concentrations below the resolution of most test kits — fish in a high-pH tank can be harmed at "0.25 mg/L" if a fraction of that is NH₃. It's the parameter that kills new tanks before the cycle finishes, and the first thing to suspect when fish in a young setup start gasping. Chronic low-level exposure is quieter but still corrosive: it damages gill tissue, blunts the immune system, and opens the door to the bacterial and fungal infections that get blamed for the death instead.
What ammonia poisoning looks like
The classic tell is fish hanging at the surface gasping, even with the filter running and oxygen fine — the gill damage means they can't extract what's there. Gills may look red, inflamed or streaky; fins clamp; colours darken or go blotchy; appetite drops and fish either go lethargic or dart erratically. Severe exposure leaves visible red patches — "ammonia burns" — on the skin and fins. Shrimp and snails are hit before you see it in the fish. There's no "too low" for ammonia: zero is the target and zero is fine.
One quiet exception cuts the other way. Many aquatic plants take up ammonium in preference to nitrate, so a heavily planted tank can mop up a small, steady ammonia input and never show a spike — part of why a densely planted setup often cycles gently. It's a buffer, not a filter, though; it won't save you from a rotting fish or a big overfeed.
How to bring ammonia down
Do a 50% water change immediately — dilution is the fastest way to cut both total ammonia and the toxic NH₃ fraction. Add an ammonia-binding conditioner (Seachem Prime, JBL Biotopol C) to convert residual ammonia to a non-toxic form for the next 24–48 hours; this buys time, it doesn't remove the nitrogen, so keep testing. Stop feeding for two days and hunt for the source: a dead fish behind the hardscape, a rotting plant, a filter you cleaned too hard, or simply too much food. If readings persist, your biological filter isn't cycled yet — wait it out, seed it with bottled bacteria or a squeeze of mature filter media, and don't add more livestock. The nitrogen cycle in plain language walks through the whole sequence, and the nitrogen-cycle timeline tool sketches how long each stage takes. If ammonia keeps reappearing in a tank that's supposedly finished cycling, why ammonia shows up in a cycled tank covers the usual culprits.
Testing notes
There are two kit chemistries and they behave differently. Nessler-based tests (turn yellow to orange) are fast but give false highs if you dose a dechlorinator or ammonia binder like Prime — the reagent reacts with the bound ammonia too. Salicylate-based tests (turn green) are the ones to trust in a tank running Prime. Whichever you use, the kit reports total ammonia (NH₃ + NH₄⁺), so always read it alongside pH to judge the real danger — the same 0.25 mg/L means very different things at pH 6.5 and pH 8. Test daily during cycling, after a deep clean, after adding livestock, or any time the tank smells off; in an established tank a monthly check is plenty. Ammonia is also the first domino in the chain, so if it's up, watch nitrite next — it's the step that follows.
