yep you got ich, the reason you did not see it on your fishis it takes a week or so to spread through the tank. dakar nailed it right on the head. put your fish in a hospital tank and treat them. and leave the display tank empty for at least 6 to 8 weeks. I would suggest quarantining the fish you buy from now on just to make sure. I am curious what was the fish you introduced into the tank that died???? Was it a type of tang?? Tangs are notorious for carrying ich. sorry to hear about the loss