One interesting thing was observed after the big insult and deaths involved in the blackout.
When I built my reef I made a support structure on PVC pipe with holes drilled in it, see way earlier in the thread. It connects to a barb fittting in the tank. When I do water changes I run a hose to the barb fitting a suck out the water and sand from inside and adjacent to the pipes. usually I get a little muddy water and sand out of there. When I did my water change after the disaster, what came out of there was EXTREMELY NASTY. And there was a lot of it. So that thing is worthwhile. I never saw bodies for my regal angel or royal gramma, they were somebody's expensive lunch. So maybe that was liquid fish.
Probably Hydrogen Sulfide from the water not moving in the pipes. See # 6 in the section, "Preventing and Dealing with Hydrogen Sulfide", in this information: