QT's should not have live rock, live sand, inverts. However, it seems like you have multiple staging tanks. Honestly, this in itself is stressful because it results in transferring vulnerable fish to new environments. Your best bet is one fish at a time in QT and, when the need arises, treat that one fish in that QT. However, if you wish to keep an observation tank free of meds, that is understandable and your choice. For fish transfer I would suggest not netting the fish but corralling fish into a container or ziplock with water and doing the transfer that way to reduce stress and injury to a fish that is actually already stressed and injured due to a disease process. In other words, do not remove the fish from water as is the usual practice when doing a transfer. Be sure to keep items used for transfers clean and sterile. Both tanks would need to have the same water conditions. Same temp, pH, salinity. This applies to the water in QT and Display as well. Keeping live rock and sand may result in contaminated fish introducing pathogens into the system where these pathogens can find a nice resting place to wait for your next fish which may not even have that disease when it is introduced. In a bare tank that has fake or non-organic objects for the fish, this is much less likely to occur. When I was keeping a reef I used a 20 gal QT. It did have about a cup of LS and a few small pieces of rubble LR. I used a few pieces of fake coral for the fish. It had filters and powerheads. Never had an ammonia problem. You do have to keep the QT cycled when there is no fish in it.
You haven't provided the previous info I requested twice already.
I think the problem that you are having, and a lot of hobbyists do have, is that they don't know what they are seeing when they encounter a problem with a fish. Then they jump right in to some kind of treatment, such a copper, without really understanding that copper may treat some diseases, but it certainly is not going to treat something like HLLE. In fact, copper is believed to be a culprit of HLLE in tangs and angels.
The pits immediately around the yellow circling the eye and the extended flesh erosion that goes beyond that is HLLE. I'm not seeing anything else in the pics you provided.
