All these diseases and such are in the ocean. If there wasn’t a natural way for fish to be resistant, all fish would be dead by now. If a fish is not stressed and has good water conditions, they will be fine. I feel it is less stressful to put them directly into a display with lots of rock (natural environment) than a bare QT tank with meds. If a fish has obvious signs of an illness, I am all for treating it first. If no signs of illness then in the display it goes. I also prefer live rock and as much diversity as I can get. I don’t like how our hobby has become so sterile!