Mass deaths are not usually related to disease. It could be the blue devil damsel took on all the fish but their would be signs of injury. Most likely it was caused by stray chemicals entering the tank, equipment failure (the carbon filter in an RO fails to remove chlorine) or, as C.J. pointed out, oxygen depredation. Since we don't have an exact cause I would fill that canister with carbon and start running it. I'd also do a massive water change if possible. Fifty percent right now and the same tomorrow if your setup allows you to make that much. I tend to rule out stray voltage as, unless the tank had a massive jolt, it would not explain wholesale deaths in the tank. Hope you don't start losing corals.