Out of curiosity, how long has the tank been running with that bioload and have you tested the water at all other than the PH? 8.4 is high, but its still within safe parameters...though I suppose a very rapid rise could cause the problem. Unless that powerhead is riffling the surface, however, its unlikely to cause a very rapid spike....
I suspect it is likely something other than the powerhead that would have caused the issue. For a powerhead to actually kill a fish...I mean...it would have to be REALLY blasting...
firefish, ocellaris clown fish
caulastrea, green star polyps, palythoa, zoas