Oh man Chin, that sucks. imo the H2S thing sounds like a shot in the dark but really may be it. Did you get the smell of rotten eggs at all? if so, H2S; if not, start looking in another direction
I find it really odd that the deaths were localized... check for mechanical problems like equipment. Do the fish have any signs of physical trauma other than flared gills? again, my condolences, hope the rest pull through.