compoundmg/L
Chlorine and chloramine
Which one your supplier uses decides whether standing water overnight is a valid preparation method. Chloramine — chlorine bound to ammonia — is stable, passes through standard carbon filtration only partly, and releases ammonia when broken, which is why a dechlorinator that handles chloramine must also handle the ammonia it liberates.
In one sentence
Both are used to disinfect mains water and both are toxic to fish and to filter bacteria. Chlorine gasses off if water is left standing; chloramine does not, and must be neutralised chemically.
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 | ≤ 0.01 | target | What treated water should read before it goes into a tank. This is the only acceptable band. |
| Any detectable residual | ≥ 0.01 | urgent | Chlorine and chloramine destroy gill tissue on contact and kill the nitrifying bacteria in the filter. There is no tolerable level in a stocked aquarium: treat the whole volume immediately. Municipal supplies commonly run between roughly 0.5 and 4 mg/L, and suppliers raise the level periodically after mains work without notifying customers. |
Practical consequences
- Always treat tap water before it goes into a stocked tank, whichever disinfectant is used.
- Never rinse filter media under a running tap. Chlorinated water kills the biofilm you spent weeks growing.
- If your supply uses chloramine, standing water overnight does not work. Check your supplier's published report rather than assuming.
- After treating chloramine, a temporary ammonia reading is expected — the ammonia portion is released and then processed by the filter.
How it is measured
- Chlorine test kit
- Supplier's published water quality report