*o2Butterfly fish are very difficult to keep for very long. Many have a very limited and specific diet. It becomes very difficult to provide that in an aquarium. Most either never eat or gradually starve to death (even if you can get them to eat flake or other food). Much of their diet consists of coral polyps which folks with a nice reef tank don't care to sacrifice.
Honestly most reef enthusiasts would prefer that this species not be collected. It's much better off left on the reef.
As far as difficult to maintain species, the list is long. I'll see what I can do to come up with one and post it on another thread.