I have 6 Corydoras sterbai in my 29 gallon, many of which are suffering from some degree of what I believe is fin rot. It first happened after a pair of Kribs spawned and I felt they got sick as a result of the stress put on them by the constant chasing.
The Kribs are long gone now and all 6 Corys are active and energetic and seem to act quite healthy ecxept for their missing fins. I treated the whole tank with a bottle of Melafix and 3 of them rapidly regrew their fins and still have them today. The other 3 showed some slight progress, but they've never fully returned.
Is there anything else I can try? It's a heavily planted 29g tank, pH around 7.2 and 0 nitrite and ammonia.