I'm not well versed in fish diseases, but I'll throw out a few ideas (been lucky for a decade except for a marine velvet outbreak on my lion of 3 years...). If you can run a UV sterilizer for a little while, that might help since you can't get the fish out (besides, if it is ich, and is showing itself, you likely have it in the tank, on the substrate, etc., already). Cleaner animals like shrimp and wrasses can also help knock out issues. I hear many have problems with cleaner wrasses, but mine has stayed fat and happy and actively harasses nearly everyone in the tank looking for something new to munch on. I introduced a small maroon for my big momma and didn't QT. The little one developed a white spot near it's dorsal that discolored the skin and started getting larger. The wrasse made short work of it, though.
If I had actually had to treat something in the past, I might be able to help you out there, but if it can be done without meds, I'm more inclined to try it. Melafix and Pimafix seemed to be helping on the lion when he somehow got marine velvet (nothing had been added to the tank in well over a year at the time...), but he eventually succumbed... That was in a 'FOWLR', though, so no corals to hurt (other than the weeds that can live in the toilet for a few days like sarcos and some zoas).