I think I just ran into the same issue. I have done salt water tanks many times in the past (mostly fish only, but a few reef). Anyhow, I had a Red Sea Max set up with lots of live rock, a few damsels left over from cycling, two percula clowns and some emerald crabs and turbo snails. I am pretty religious about water testing. A couple days ago I added a few more emeral crabs and snails. The next day, ALL the fish were dead. I immediately removed them and also tested the water, and was surprised that everything was in pretty much perfect shape (with pH a bit lower than I want... 7.9, but still within spec). I confirmed with the pet store (whom I have been using for multiple decades) that the no copper was added to the tank where the snails/crabs came from. The snails/crabs all seem fine and are eating just fine. All fish were fine on their "last night" and during their "last supper". The only thing I have noticed yesterday is a slight film on the water surface which might indicate the same oxygen depletion. Does this sound right? I can't think of anything else that could cause this with my water condition so well. (no cleaning agents nearby, etc). I just don't want to add more fish if there is something else (and it isn't fair to the fish!). I guess I was wondering if the snails/crabs are similar to corals in requiring less oxygen... lending more support for an oxygen depletion issue, which might be resolved by a simple re-aim of one or two of the power heads.