Fin rot is one of the most common problems in home aquariums. The fins look ragged, split or shortened, often with white, black or red edges. It is usually a bacterial infection that takes hold when fish are stressed.
Common causes
- Poor water quality: ammonia, nitrite or high nitrate from overfeeding, overcrowding or too few water changes.
- Stress from sudden temperature changes, shipping or a new tank.
- Injuries from fin-nipping tankmates or sharp decorations.
Prevention
- Test the water regularly and do partial water changes on a schedule.
- Avoid overcrowding and overfeeding.
- Choose peaceful tankmates for long-finned fish such as bettas and fancy goldfish.
- Quarantine new fish before adding them to your main tank.
Treatment
- Fix the water first. Mild fin rot often stops once the water is clean and stable, and fins slowly regrow.
- Watch for progress. If the damage keeps spreading after the water is right, or reaches the body, an antibiotic may be needed.
- Treat in a hospital tank with a product labeled for fin rot, such as Fix Flox, Fix Mox or Fix Flex, following the label and finishing the full course.
Fins regrow over several weeks; new growth is often clear at first. If the fish stops eating, the rot reaches the body or several fish are affected, ask an aquatic veterinarian.
Fish antibiotics are for ornamental aquarium fish only: not for human use and not for food-producing animals. This guide is general information, not veterinary advice; for valuable or persistently sick fish, ask an aquatic veterinarian. Last reviewed: November 2026.