Do you think the water might be too clean? With my careful observation of a similar situation.
If there are no biologicals in the tank what are the corals going to eat?
I had this same problem a few months back. I noticed lots of bleaching and die off. All my temps through out the day were fine I keep a constant temp of 74 in my tank. I did notice I stopped for a period of about 6months of feeding at night with cyclopeeze, I generally feed once a month but with school and work I was just to tired to feed at night and figured there were plenty of things the corals and zoas could munch on. About two months ago I started my monthly night feeeding routine and I have been noticing coral growth and overall polyp extensions on everything. I lost a lot of zoas (blues, bam bams, olivies..... ) In the end I realized I had too pure and clean water zero nitrates is not actually a good thing, your tank requires a low level of nitrates and nitrites and a dap of phosphates to let phytoplankton grow in order to feed the corals.