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 planaria guide guide makes the numbers obvious.
The treatment
Step by step:
- Move the sick fish to a hospital tank if you can, see the best fish for a 10-gallon tank (2026).
- Raise the temperature slowly to 28 °C to speed the pathogen's life cycle (species permitting).
- Add 0.3% aquarium salt, dissolved first, and keep it stable with a 34% 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 Red pest 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.
