Firefighter,
I think the most important symptom you reported is that while fish are dying, the more delicate corals and inverts are fine. This completely rules out all water quality issues, copper, temperature, pH, etc.
The only problem it can be is a fish disease. Now, as to what disease it is, there is not enough to go on in the way of symptoms. Its been my experience that there is only one common disease that will take out most/all of the fish in the tank, yet not give major symptoms....Amylodinium, a gill parasite. People who have run into it before will usually see the subtle symptoms, but others may miss them: rapid breathing and a *slight* dusty, cloudiness to the fish's skin.
Another often missed disease are trematodes, (flukes). Again, the symptoms will include rapid breathing, but fish will also show tattered fins, cloudy eyes and scratching. In these cases, the fish may die if not treated, but the deaths will be spread out further - like one every week or so.
Are there any fish remaining? If not, it was likely Amyloodinium.
Jay
