I don't know if this fish can be cured, but having it in a hospital tank is a start. One, keep a close eye on the ammonia level, as a smaller tank changes faster, and if ammonia is detected, bind it with Safe, Prime, or Cloram-X, and use a SeaChem Ammonia Alert Card to assure the ammonia is not toxic ammonia. If toxic, retreat with the binder. Two, get the temperature up to close to 75F(25C) to speed the healing process and raise the salinity to 0.8%, about 7 pounds per 100 gallons to take pressure off the kidneys due to the damage to the skin allowing increased amounts of water through the membrane called skin. The antibiotic should help, but the key now in addition to the antibiotic is time, temperature and extremely good water quality.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard