If you only learn one thing about Chlorine poisoning, make it this: the visible symptom is rarely the real problem. The good news: most cases are treatable at home if you act in the first few days.
The symptom
Frantic swimming and clamped fins minutes after an untreated water change.
What is actually causing it
Ranked by likelihood:
- Stress from poor water — ammonia or nitrite above 0 ppm, or nitrates over 40 ppm.
- Secondary infection — a wound or parasite opens the door.
- Temperature swing — more than 2 °C in a day weakens immunity.
Test the water first; treat the cause, not just the spot. A our canister vs hang-on-back filter: which should you run? guide makes the numbers obvious.
The treatment
Step by step:
- Move the sick fish to a hospital tank if you can, see our why is my fish gasping at the surface? 6 causes and fixes guide.
- Raise the temperature slowly to 29 °C to speed the pathogen's life cycle (species permitting).
- Add 0.1% aquarium salt, dissolved first, and keep it stable with a 28% water change every other day.
- Follow the medication label exactly; overdosing does more harm than the disease.
Prevention
Quarantine every new fish for 4 weeks, keep nitrates under 20 ppm, and avoid overcrowding. Stability is the best medicine, a tank that holds its parameters rarely sees outbreaks.
Plant-tank considerations
In a planted tank, treat Chlorine poisoning carefully: some medications harm sensitive plants and invertebrates, so a separate hospital tank protects both. Keep flow gentle so the sick fish is not stressed further.
