Finrot

[edit] What is Fin Rot?

Finrot is a common bacterial disease of fish kept in low quality water conditions.

It is caused by gram negative bacteria like Aeromonas, Pseudomonas, or Vibrio which eats the delicate membranes of fish.

It is easily treated and the fin will grow back if not too badly corrupted.

Bad water - regular changing of the tank water and lowering of wasted fish food will prevent the levels of bacteria from climbing.
Fin damage due to overcrowding, handling, other fish.

Regular water changes can often cure this if it's not too advanced.
Adding a level of salt to the water is a good mild treatment if the species of fish can tolerant it.
Antibiotics (not available in Europe over the counter) are an option for advanced cases. Chloramphenical, Oxytetracycline, and Tetracycline for example.
Commercial finrot treatments of which there are many.