ok, possibilities that i can think of:
1, something has changed in your water that has caused a reaction
2, he's caught his tail on the net and a severe case of fin rot set in very quickly (not sure if it would kill a fish)
3, something has been transfered into the tank on the plant that has infected him (absolutely no offense meant to HaNs, just speculating here)
4, he was already infected with something that just took a while to take affect
5, a combination of any of the above
i know its not a lot to go on, but it's a start. keep a close eye on the young one for any similar holes in the fins.
as for any sort of treatments, i wouldn't know what to use as i really have no idea what could have caused the sudden death.
hope this help