When they start droping like flies
Why are you asking this question?
Are your corals dying?
If you are losing them then your corals are sick
If you see the flesh of the polyps start to receed from the skeleton....your corals are sick or starving
If you see brown jelly over the polyps of the corals...your corals are confirm dead, no way to save them
If your mushroom did not open up for a week then it is sick
If your mushroon start to rot....then it is dying
Most of the "sickness" is cause by poor water quality, aggression, lighting or parasites.