Maroon Clown with abdominal swelling

Maroon clownfish approx 3.5 years old in 4 year old established 75 gal reef.
Soft corals, anemone, inverts etc all thriving. No recent additions to the
tank in many months and the fishload is low, all others look well and are
eating like pigs. Auto dose kalk and keep salinity approx 1.023 and steady.
Temps approx 77 deg F.
Clown hasn't been eating lately and respiration is a bit labored. Noticed
some slight swelling and discoloration last night on left side abdomen.
Tonight its more pronounced. Fish still appears to be swimming normally but
tomorrow night its going in a hospital tank.( The kids are using the spare
tank for their hermit crabs). From the research I've done, I'm thinking some
sort of bacterial infection. It dosn't appear to be brooklynella, but in my
4 years of marine keeping, I haven't come across anything like this. I keep
3 other tanks and keep the maintenance steady every week.
Any suggestions as to an antibiotic or other treatment. We hate to lose
these little guy's

Thanks
Chris.