Hi Muddy, I am very sorry for your loss... as you surely realize, butterfly fish are very beautiful and can be very difficult to keep.
To help you have better success in the future I wanted to ask you some questions and give you some helpful hints.
First of all, every butterfly I have ever kept has been omnivourous with the majority of the diet consisting of vegetation (Dried Seaweed).
Secondly, how closely do you monitor your water values? If you have a coral, your water quality must be supreme or that would've surely died first. But for some reason Butterflies seem to be more sensetive to nitrates than many other
SW fish I have kept.
My guess is that the diet was the main culprit. I have made it habit to become an "expert" on every fish I have ever gotten, that way I have very few questions when introducing them to my tanks. I also find it very intreguing to hear others' experiences with certain fish, since they many times vary in extremes. For example, I had a puffer and a neon goby who lived harmoniously together for over 6 months. The puffer would eat live foods and leave the goby alone. So I added another goby and i don't think he touched the water before the puffer ate him... but the puffer still left the other goby all alone.
Hope this advice helps, happy fish keeping!