Fish & shrimp health guide
Fin Rot
Also called tail rot, fin melt
Fin edges going ragged, milky or red, eating inward toward the body.
TL;DR, Fin Rot
Fin rot is a bacterial infection that takes hold when water quality slips or a fish is stressed or nipped. The fin edges fray and discolour and, untreated, the rot eats back toward the body. It is not really contagious, it is opportunistic, so the real fix is the water, not just a bottle. Clean water and good food reverse mild cases on their own; advanced cases with reddened, receding fins need an antibacterial treatment. Long-finned fish like bettas and angelfish get it most.
What you see
Fin edges look ragged, torn or frayed, often with a white, milky or reddened margin. The fins shrink over days, and in bad cases the rot reaches the body and the fish becomes lethargic.
What causes it
- Poor water quality, high ammonia or nitrite, or overdue water changes
- Fin nipping from tankmates leaving damage that gets infected
- Chronic stress from crowding, bullying or unstable parameters
- A cold or dirty tank lowering the fish's resistance
How to treat it
- Test the water and fix the basics first: ammonia and nitrite to zero, nitrate down with a water change.
- Improve maintenance, clean water is the single biggest lever on mild fin rot.
- For advanced or reddening cases, use an antibacterial or antibiotic treatment suited to the fish.
- Remove the fin-nipping tankmate if that is the cause.
- Feed well, since good nutrition speeds fin regrowth, which grows back clear at first.
Treatment safety in a planted tank
Many antibacterial medications knock back your filter's beneficial bacteria and can stress shrimp and snails. Dose in the display tank only with an invert-aware product, or treat the fish in quarantine, and re-check ammonia during and after treatment.
Prevent it
Keep up water changes, avoid known fin-nippers with long-finned fish, and keep the tank stable. Most fin rot is a water-quality problem wearing a costume.
Easy to confuse with
Physical fin damage from nipping or sharp decor looks similar but does not spread or discolour once the fish is left alone. Columnaris moves much faster and often hits the mouth and body too.
Species most at risk
Species that catch this most often, or that need a gentler treatment:
- Siamese Fighting Fish — Long, heavy fins make it the classic fin-rot patient, especially in unheated or dirty tanks.
- Angelfish — Long trailing fins are easy targets for nipping and infection.
- Guppy — Fancy tails fray quickly in poor water or with nippy tankmates.
- Pearl Gourami — Long ventral filaments are prone to damage and rot.
Frequently asked questions
Direct answers to the questions search engines and AI assistants surface most often about this species.
Will fin rot heal on its own?
Mild cases often do once the water is clean and the fish is unstressed. Fins regrow, clear at first, then re-colour. Advanced, reddening cases need medication.
Is fin rot contagious?
Not really. It is opportunistic, so other fish get it only if the same poor conditions or stress affect them too.
Why does it keep coming back?
Because the underlying cause, usually water quality or a nipping tankmate, has not been fixed. Treat the tank, not just the fish.