It's just a guess. I took a quick look through my Basslets, Dottbacks, and Hawkfishes book by Scott Michael and the closest cardinal I can find that resembles the one in your picture is called the Halfbanded Cardinalfish, Apogon semiornatus.
poor fish was swimming, eating and all...tens minutes later it was floating upside down? Don't get it I saw it eat 10 minutes before. Oddest death in my 17-18 years with saltwater tanks.