does it look beaten up?
liek raggedy fins, and the lille side fins look like stumps, or are getting to that stage...
sometimes angels can be harsh.
does the other angel go all rigid and stiff and jab at this angel?
if so then its probably not a disease, just been beaten up.
salt will help. you dont need to put a heap in, just a bit to stimulate the growth of slime coat will do. that and stress coat helps. aloe vera gelly is good too.
a pic would help, because if by some remote chance, it is a disease id feel very bad about giving wrong / inappropriate advice based on the context.