It's been reported by many with issues similar to the OP's that low nutrients and sudden increases in light intensity stress corals. The duration of the stress lends to the decline to the point of the bad bacteria (Vibro) attacking their tissue. Infection sets in. Limited protozoa surrounding the coral now proliferate and consume the dieing tissue. Thus RTN. Once this happens, fragging the dying tissue off and removing it from the system aides in reducing this infection spread to near by healthy corals. In the case of a system wide event, total loss is highly likely. Correcting the conditions that caused the stress helps, but once infection sets in, it can take months to recover.
Mature systems seem, this is anecdotal, to fair better in these stress events. Healthy corals seem to handle stress events at first.
Now that I see you present it this way, it does seem to be a very common theme particularly in the SPS world. It is often hard to pinpoint with certainty, the initial stressor(s). While it is hard to resist, this pattern really does support the repeated mantra of "if it ain't broke...". I wish I were better at following that advice sometimes. OK, a lot of the time.
It has taken me awhile to appreciate the breadth of life forms in our systems. Each are lying in wait for the parameter change that suits their particular physiology. They are all there, all the time, but are held in check by the balance of the ecosystem. Bacterias, protozoan, algaes, herbivores, parasites, pests, working fish, planktons, corals. Each have a means of barely subsisting quietly, loitering for their moment to take over.
I apologize for getting all philosophical; it is just that I have come out of 3 months of dinoflagellate Hell. And while I can no longer detect them, I know they are there. Resting for now. Just waiting for me to add that super cool PAX Bellum algae reactor that would look fabulous next to my sump. Oversized for sure.
@mfollen I am sorry your your beloved SPS are under attack. It is obvious you have the chops for this hobby, so I am not worried about you restarting if that is where you ultimately have to go. Hopefully the system steadies itself over the next few weeks.
