Odd fish death

[fishoholic]

So I came home last night to find out that my rainford goby and one of my anthias had been found belly up floating in the tank. The rainford goby was in our frag tank the water tests came back normal and the anthias was in the main tank and tests came back normal on that tank too. Both fish were fat and seemed to be eating fine and looked perfectly healthy that morning. When the fish were found dead they hadn't lost any of their colour or body size and there was no nip marks on the anthias (the rainford was the only fish in the frag tank) Anyone else ever had a fish go belly up on them when the fish seemed and looked perfectly healthy still, even in death? Any ideas as to what could of happened? Since water quality doesn't appear to be a factor, I'm guessing it's something else.