Ammonium
Ammonium and ammonia are the same nitrogen in two forms, and which one you have is decided by pH. Test kits do not distinguish them. The distinction matters twice: for toxicity, and for planted tanks, where ammonium is the nitrogen source plants use in preference to nitrate.
In one sentence
Ammonium is the charged form of ammonia that dominates in acidic water. It is far less toxic to fish because the charge stops it crossing the gill membrane easily, and it is the form plants take up preferentially.
How to read a measurement
Bands are guidance for interpreting a reading, not thresholds with sharp edges. Sensitivity varies by species and by how long the exposure lasts.
| Band | Range | Verdict | What it means |
|---|---|---|---|
| Not detected | ≤ 0.02 | target | Normal for an established tank. A test kit reporting zero total ammonia means neither form is present in a measurable amount. |
| Trace total ammonia | 0.02–0.25 | watch | How serious this is depends entirely on pH and temperature. At pH 6.5 most of it is ammonium and the fish are in far less danger than the same reading at pH 8.5, where a much larger share is free ammonia. Use the free-ammonia calculator rather than the number alone. |
| Elevated total ammonia | 0.25–1 | act | Water change now, stop feeding, and check pH before doing anything that would raise it. Raising pH converts ammonium into the toxic form and can kill fish that were coping. |
| High total ammonia | ≥ 1 | urgent | Large water changes and increased aeration. At alkaline pH this is an emergency; at acidic pH it is still a filter failure that needs correcting. |
Why the difference matters for plants
Aquatic plants take up ammonium in preference to nitrate because assimilating nitrate costs energy to reduce it first. This is also why a spike of ammonium is strongly associated with algae blooms — algae exploit it faster than higher plants can.
Reading a test result
A standard ammonia test reports both forms combined. To know how much of it is the dangerous form, you need the pH and the temperature. AquariumHQ's free-ammonia calculator does that conversion and shows the formula it uses.
How it is measured
- Liquid reagent test kit — measures TOTAL ammonia nitrogen, meaning free ammonia and ammonium together, not the toxic fraction alone
- Test strips, which are less precise and share the same limitation
- Free-ammonia badges and electronic monitors, which respond to the un-ionised fraction specifically and therefore answer a different question from a reagent kit