With all due respect to .chris. improved coloration is unlikely. Cyanide caught fish tend to "exist" in a mindless state just sort of hovering around. They will not eat, they will not hunt, just sort of hover. Collecting tangs with cyanide is unlikely as they are an open water fish and it would be virtually impossible to dose them with cyanide. Mandarins, however, are highly localized and cyanide collection is common as they can harvest multiple ones.
I seriously doubt that this is the issue.