Mass Coral Death

[WarDog]

Do you have a second tank (QT) you could set up for your primo livestock whilst you figure this one out? Maybe a fellow CanReefer in town?
Perhaps something is dead behind your rocks? Also live rock doesn't last forever, it's always a good idea to add/change out a piece or two every year or so. Keeps diversity in the good critters.
I still recommend a RODI system, this would only be good for you and your livestock.
Take some water to the LFS and have it tested if you don't have a master kit.
If it were my tank I would be going crazy right about now, as I'm sure you are! GOOD LUCK!