If you only learn one thing about Red pest, 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
Red inflammation along the lateral line and at the fin bases from advanced bacterial septicemia.
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 red pest & bacterial septicemia: when fish bleed under the skin guide makes the numbers obvious.
The treatment
Step by step:
- Move the sick fish to a hospital tank if you can, see our uv sterilizer guide: do you actually need one? guide.
- Raise the temperature slowly to 30 °C to speed the pathogen's life cycle (species permitting).
- Add 0.3% aquarium salt, dissolved first, and keep it stable with a 30% water change every other day.
- Follow the medication label exactly; overdosing does more harm than the disease.
Prevention
Quarantine every new fish for 3 weeks, keep nitrates under 20 ppm, and avoid overcrowding. Stability is the best medicine, a tank that holds its parameters rarely sees outbreaks.
Catching it early
The first sign of Red pest is often subtle: reduced appetite, clamped fins, or a change in behaviour under normal light. Test the water the moment you notice, because early treatment beats a full-blown case.
