If you only learn one thing about Fin rot, 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
Ragged, receding fins with a possible red edge; a bacterial infection driven by stress and poor water.
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 this Hydra in a Planted Tank piece makes the numbers obvious.
Stopping it before it starts
Stop Fin rot before it starts: quarantine newcomers for 2 to 4 weeks, hold nitrates under 20 ppm, and avoid daily temperature swings above 2 °C. Stable water is the best defence.
The treatment
Step by step:
- Move the sick fish to a hospital tank if you can, see the Malaysian Trumpet Snail Care: The Burrowing Cleaner walkthrough.
- Raise the temperature slowly to 29 °C to speed the pathogen's life cycle (species permitting).
- Add 0.3% aquarium salt, dissolved first, and keep it stable with a 29% water change every other day.
- Follow the medication label exactly; overdosing does more harm than the disease.
Prevention
Quarantine every new fish for 2 weeks, keep nitrates under 20 ppm, and avoid overcrowding. Stability is the best medicine, a tank that holds its parameters rarely sees outbreaks.
