Fin rot is nearly always a sign of stress and the most important step is to try and determine why the fish is stressed and resolve the problem.
It is important to realize that the stress could be initiated by virtually anything, for example poor water quality, parasites, etc. – so therefore it really needs a full examination of both the fish and the water . If the problem is caught early enough, simply removing the source of the stress will be enough and the fish will probably recover. However, if the problem is more severe and the infection has a firm hold it may be necessary to trim away the diseased portion of fin and consider a course of antibiotics. Obviously this may require professional assistance.