A while back, I had let one of my smaller tanks (just LPS and fish) get rather mucky. The usual algae, detritus, coralline, but also some chrysophyte like stuff that looks like dinos but it is not. Sometimes called "golden algae".
Anyway, I finally got to work with a scraper cleaning the sides and bottom of a 40 breeder. I turned off the circulation so I could vacuum out all the crude into a sock in the sump. The fish were hiding. LPS pretty annoyed. This took about 20-30 minutes or so and then turned circulation back on.
Come to realize then that my fish weren't hiding. They were all dead bar one small tomini tang.
Somehow, stirring up all that muck either:
a) Consumed all the oxygen very quickly
b) Released some kind of serious toxin
c) Somehow that level of turbidity suffocated the fish.
d) All above or other
In a "normal" state, those fish could last hours without circulation.
It is an imperfect comparable to your situation, but it has changed my risk assessment protocol for sure.