I went back and re-read your other thread, to see if anything stood out. You changed bulbs, as well as ran carbon around the same time; I'm wondering if the carbon really cleared the water to the point of an increase in PAR enough along with the new bulbs to shock some corals. There was also mention of flatworm exit in that thread. Did you ever end up using that to treat planaria? If so their die off is pretty toxic in a large quantity. Have you tested for Nitrate at all? one other thing that just came to ming was CUC; any sandsifters that could have moved some sand that has been un-disturbed for some time?
I'm still leaning toward Alk and the stability of parameters to be the culprit here. But a combination of all the above could have a hand in it as well. How are things looking today? better, same, worse?
Very interesting comments regarding the par. Can cleaning water with carbon affect par that much? FYI I did replace bulbs slowly over a month period to try not shocking the corals too much. Regarding flatworms I never got around to running few exit. My plan was to eradicate as many as possible first naturally with the nudibranch. Nitrate was tested at 0 last night before my water change. It is possible I stirred up some of the sand bed recently moving some frags and repositioning my clam. That's a possibility.
Today things look better I suppose, nothing new appears to be dying at least lol. Toadstool looks a little better as well, no skin shedding but it's still not 100% expanded to its full size. My large Duncan colony has me slightly concerned as its fully retracted, but they are known to do that from time to time so we'll see how it looks over next few days.