Why this reading is so dangerous
Unlike pH or temperature, there's no "acceptable range" here — the target is always 0 ppm. Ammonium itself is relatively low-toxicity at neutral pH, but as pH rises it increasingly converts into the far more toxic ammonia (NH3), which damages fish gills directly.
In an established aquarium, ammonium is normally broken down immediately by nitrifying bacteria into nitrite and then nitrate. The problem almost always occurs in newly set-up tanks, where this bacterial colony hasn't established itself yet — hence the term "cycling phase."
Thresholds at a glance
| Reading | Classification | Action needed |
|---|---|---|
| 0 ppm | Normal | none |
| 0.25 ppm | Warning | check the cause, water change |
| 0.5 ppm | Critical | immediate water change |
| >1.0 ppm | Acute danger to life | emergency steps, see below |
Why the reading rises
The most common causes:
Newly set-up tank: The nitrifying bacteria in the filter haven't colonized sufficiently yet (typically takes 4–6 weeks).
Overfeeding: Uneaten food decomposes and releases additional ammonia.
Stocking too fast: If a tank is stocked with too many fish too quickly, waste output exceeds the filter bacteria's capacity.
Filter failure or over-cleaning: If filter media is rinsed too thoroughly under tap water, the bacteria die off and breakdown briefly collapses.
Dead animals or plant matter: Unnoticed decaying material in the tank releases large amounts of ammonia.
What to do about elevated readings
Once ammonia or ammonium is measurable: act immediately, don't wait.
Immediate water change: Change 30–50% of the water to dilute the concentration right away.
Stop feeding: Skip feeding for 1–2 days to avoid adding further load.
Find the cause: Look for and remove dead fish, decaying plant matter, or leftover food in the substrate.
Add bacterial starter: Special filter-bacteria products can speed up cycling, but they don't replace a water change.