Sorry to hear that I hate loosing fish! I lost a female dragons blood a similair way fine at night then the next day dead.. Got another one recently and lost that one due to stresses in shipping!
I was thinking it may have been bloat but its usually the later stages they stop swimming around and if he was eating last night no luck.. Hope everything else in your tanks alright and you can find another one if he was needed for breeding etc, I think I saw one for sale on trademe?