It's not the size of the fish when it comes to saltwater fish its the amount of activity and space they require to get the needed activity and "exercise" if you want to call it that. Not saying that it was the tank size that caused the rapid downfall, but the tank was still too small. I don't think it was the tank size guys that did him in, even though the tank was and still is too small for any tang.
Even though RC is such a great place with a lot of diversity among folks, I don't think anyone on this site besides other's that keep tangs in too small of a tank will say that your tank isn't too small for tang. Even though tang's need nori and I feed my Powder Blue nori each day, I also don't think this had much to do with it's rapid downfall. Yes maybe stress from cramped quarters, not enough algae and other nutritional deficiencies can all combined be the downfall of a tang, but not overnight. Keep searching and watching the tank as it could happen again whatever it was.
I am in no way saying it's right to keep any tang in a small tank, and anyone that knows me on the boards knows that I am one of the first to tell people that their tank is too small for a tang, but I don't think this had anything to do with his death overnight.
That being said, keep watching the tank as it could happen again to your other fish if you don't figure out what it was. Have you checked for stray voltage or if any contaminants could of made there way into the tank? Tangs are one of the most sensitive fish and his/her death could be an indicator of more to come. Keep us posted.