Are you sure its fin rot and not just torn fins? Fin rot gets steadily worse quite quickly but torn fins should heal within a week or two @82F.
Either way, the best treatment is daily water changes and gravel cleaning, without this even the best antibiotics will have a hard time keeping it at bay.
Whenever my angels have fin trouble I start doing 10% daily water changes and if its bad add 3/4 dose of melafix. Its best not to dump a full dose in on first day of treatment, gradually work up to full dose over a number of days.
I would only consider antibiotics if it gets so bad as to have the rays exposed near the body, otherwise the treatment above should give results within a week.
Ken