Recognizing symptoms
Early on, only small tears or a slightly frayed edge on the fins may be visible. Left untreated, entire fin sections can rot away over time. The related mouth rot shows the same pattern on the mouth area, which lightens and frays.
Why fin rot is almost always a water-quality issue
The responsible bacteria are present in practically every aquarium — they only become a problem once fish are weakened by poor conditions. The most common triggers: overstocking, overfeeding, and infrequent water changes, which can lead to elevated ammonia or nitrite readings.
Treatment
In the early stage, consistent water hygiene is often enough: a generous water change, thorough tank cleaning, and if needed a gentle filter cleaning to lower bacterial load.
For more advanced cases, antibacterial medication from a pet store is also needed — check the product label and, if in doubt, consult experienced fishkeepers or an aquatic veterinarian.