Mike this happened to me a couple of weeks ago with my small yellow tang. Was swimming around just fine, ate the same day, no signs of any sickness. Found it stuck to my koralia 3.

My guess butterflys like tangs have a large flat surface area and if they get to close the pump can easily suck them to it. Unlike wrasses, basselets, hog fish etc. with long narrow bodies.

Sorry for your loss.