I've been advised by a mod when I was treating my fish that marycyn is hard on the kidneys. I had one person tell me she cured her fish with fish mox. However, she did say it came again later (not immediately repeat episodes.)
Maybe this will help you decide I know reading is the last thing on your mind but glance through it:
I probably already posted this link for you. Most of it is good. The only advice I didn't think much of was add ingAQ salt to a medication mixture which would seem contradictory if a fish may have kidney issues.
I am up late and have to go to bed. I hope he improves.