fin erosion

Greetings all, I have 10 gallon fowlr cycled it for 30 days and added a blue damsel after 4 minutes of fw dip he was heavy breathing and got normal ate and started digging the substrate all of a sudden he was on his side with fins and gills are torn up he was heavy breathing and eventually...

I have a scopas tang that I got about 2 years ago and ever since then his fins have been eroding, I’ve tried pimafix, melafix, prazipro, copper power, vitamin supplements, none of those work. All the tank specs seem to be on point, all the other fish are happy and healthy, I feed him nori...