I assume you use RO
and that should have isolated you from any changes in the city water supply.
If it was stray electricity, you would probably feel that when immersing your hand in the tank. It would need to be pretty strong to effect the fish as they have nothing tying them to a ground source.
It could be a pathogen, but most of them would leave visible signs like inflamed gills or scale damage.
Food source should also be looked at. A 02 crash could suffocate the fish and leave the corals alone for short periods. But you would see that in their extended gills.
A toxin like hand lotion or soap could raise havoc. Those are all the ideas I have to share. Good luck!
Sent from my iPhone using Aquarium Advice