Same thing happened to one of my green chromis a few months ago, and they've got a much smaller cross-section than a tang or butterfly (although not exactly the same emotional cost when it goes belly-up). No sign of disease or injury to mine, either, and it had eaten well just that morning. Found it stuck to a K4 that evening. Until I have the time to go back to school and get my marine biology degree so I can learn how to do necropsies (yeah, right...), deaths like this will remain terrifically frustrating and hopefully rare.