The short answer
Fin rot is a bacterial infection that eats away at the edges of a fish’s fins, leaving them ragged, frayed or discoloured. It’s almost always secondary to poor water quality — the real problem is usually ammonia, nitrite or dirty water weakening the fish, with bacteria taking advantage. So the first and most important step is to check and fix your water, not to reach straight for medication.
How to recognise it
Look for fins that appear torn, milky, or edged with white, black or red, often getting shorter over days. It can be mistaken for simple fin damage from decor or nipping — the difference is that fin rot progresses and the edges look inflamed or disintegrating rather than cleanly split.
If only the tips are affected and the fish is otherwise active, you’ve likely caught it early.
The safe first response
For most early cases, the treatment is clean water: test, do a water change, and keep conditions stable and warm. See how to do a water change. Good filtration matters too, since it’s what keeps ammonia and nitrite at zero — browse our aquarium filters hub if yours is undersized.
For stubborn or fast-spreading cases that don’t respond to clean water, a proper antibacterial treatment may be needed — but research the right product for your fish and tank, and ask a vet or experienced keeper before dosing. This is general guidance, not a diagnosis.
Prevent it coming back
Fin rot rarely appears in a well-run tank. Keep the tank cycled, do regular water changes, avoid overcrowding, and reduce fin-nipping by choosing compatible tankmates. For the bigger picture on avoiding disease, see how do I prevent fish disease? and why does my fish have a torn fin?