I can only say what my experience was. I came home from work to find my favorite cichlids dead. There were about 15 that were still alive (the smallest ones) but gasping at the surface. I immediately redirected one spray bar and hooked up one of the HOB. Within half hour all surviving fish were breathing normally.
Every tank situation will be different. My fish certainly had not started decomposing.
Every tank situation will be different. My fish certainly had not started decomposing.