I don't know it it was coincidence, but I had frozen food defrost and refroze it (I don't know how long though), and fed the tank. I has a maroon clown, red sea sailfin tang, and yellow tang all die the same day. I had those fish for about 4 years.
Since other people have had not had an issue with it, maybe it was defrosted too long. But I'm too hesitant to refreeze something again.
Btw, killing off a maroon clown isn't easy. Generally you need something on the order of a 357 magnum.
Ken