Might be worth dosing a little Prime, perhaps you have a little chlorine or chloramine contamination from cleaning something? That would totally explain the invertebrate death and corals being irritated.
I do have prime on hand. So I can dose prime with no negative effects? (As long as I follow directions for dosage amount on the bottle)
Also, how are you checking your salinity? A good refractometer is the best way. If your salinity is off by more than 2ppt from your local fish store, that would also explain the issues.
I use a refractometer also bought calibration fluid to make sure the refractometer was calibrated correctly. (currently at 35 ppm / 1.026)
Finally, what's the temperature in the tank?
Temperature in tank is 76 - 77.