[Reef_Geek]

First, glad to hear all is well now.

Just a shot in the dark... copper sulfate is toxic to both fish and protozoa. The idea is that it's going to kill the protozoa before it is lethal to fish. Copper sulfate is known to cause damage to fish gill tissues, and its effects can vary from specimen/species to the next. My bet is that the fish was "ram ventilating" to increase water flow over its gills.