8.5 is high but not high enough to take out corals... It's something else.. My ph has been 8.5 the past 7 days with no ill affects.. I'm happy it's that high...
Check you alk and also if it moved a lot, like lowered or went up alot in the last couple of days... Also how old is your tank... Most established tanks don't go that high without some serious help..
Phosphate at 0 could be the problem but I doubt it's at 0 unless your using to much phosban or other export.. If it drop to 0 in the last couple of days that would also do that to acros..
76 to 80 is a big swing in temp how are the fish??