Hi, not a frequent poster, but I wanted to add my 2 cents. I have a tank with a semilarvatus, miliaris, falcula, and auriga butterfly. I have only been doing this for three years, but I've noticed that the butterflies seem to have more trouble going through quarantine than other fish. Maybe it's the lack of live rock to graze on? A while back I made the dangerous decision of not QT'ing my butterfly fish. I have not lost one since. There are a few products that can be used to treat Ich and some other infections in the DT while your inverts are in there as well. They may not be as effective as copper etc, but I am willing to take that risk. Many would consider that folly, I know. I only purchase from DD and I believe that decreases my chance of getting an infection in my DT.
I also periodically feed corals to my fish! Yes, it's true. I keep green star corals in the refugium and every week or so cut off a piece and drop it into the tank. The butterflies love it! You can also go to the LFS and buy $5 frags once or twice per month and drop them in. It is certainly no more expensive than some of the other things we do in this hobby, and I believe it might help the butterflies to thrive, since it is more like the natural food of a lot of them.
BTW NONE of my butterflies have ever ignored a single coral I have put into the tank (ricoridia, mushrooms, acropora, fungia, various brain corals) except for one leather coral that has been in there for about one year.
Bruce