Very sorry to hear this Jim (and Ross, sorry to hear of your tang)
Your observation about the gills concerns me .. That doesn't sound like something a damsel could cause??
This sounds eerlily reminiscent of when I lost nearly all my fish in my ritteri tank a few months ago. One by one, they suffocated over the course of a few hours.
Anyhow, I know how discouraged you must be feeling right now. Hang in there.