Corals are actually hardy creatures that can take all kinds of abuse temporarily (inluding sitting in a stainless steel sink for 3 hours and then having a cold coffee bath, can anyone say acidic
) but the key word is temporary.
It's making them live under foul conditions for a long time that does them in. They are wild animals and like all wild animals they can't afford to "look sick" or they will be eaten so they "look" healthy until they are just about dead