FEED, FEED, FEED. fat fish are healthy fish, that's been my experience. Big temp swings need to be stabilized, and keep good water parameters . Then hope for the best. Recently brought in a blue hippo and yellow tang from a tank breakdown. Both fish had ich, tossed them in and proceeded to feed them and fatten them up. No more signs of ich and other fish a still healthy. Have done this in the past with other unhealthy fish that had ick/ HLLE from hobbyists that let there tanks go.
This did not work for Uronema or Brooklynella, the infected fish ended up dying, however the existing inhabitants did not get wiped out as well. So, the lesson learned there was even if a LFS says all the fish have been quarantined and are healthy doesn't mean anything.