What damsel is it? Cromis are very fragile.
Assuming you've a ball valve on your plumbing, open that and see if you've a dead fish fired into the sump.
Also no marine fish float when they die. They stay down for food. Some freshwater fish float, as birds can eat them, which is less likely at sea. No idea if that's by design but makes sense. Depends on what their swim bladders decide on death.