I've had a couple of events that caused my water quality to change significantly and pretty rapidly. In both instances, it was the effect on the corals that alerted me that something was amiss. What I found was that by bringing my parameters back into check fairly slowly, I was able to minimize damage. Even still, it took a while for the corals to respond. It seems to me that any large, rapid change can cause more harm than good. Most corals are somewhat resilient. Given enough time and trying to maintain a consistent balance once parameters have been reestablished can produce results even in SPS that appear to be goners. But it takes time. I kind of monitor my coral health to a certain degree by Alk consumption.