[Nate]

I would be willing to place a large bet on low oxygen levels. Buildup of protein on the surface...usually happens when someone is not using a sump, and an inadequate hang on skimmer (i dont know what skimmer you have/had), and little to no powerheads pointed up at the surface. Did you have a slimy coating on the surface of your aquarium?

Nate

However this is not caused directly by the adding of fish, it would happen eventually if you just had 1 fish. But the adding of new fish increased your need for available oxygen, with that, the new fish are stressed and their oxygen uptake is thus increased, and in the end it comes to a point where no one can breathe and they die. Good thing is your corals should be fine, I have seen them handle this no problem