Sorry to hear Kevin...the mysterious unexpected loss or losses leaves us wondering and keeps us wondering what/where we went wrong. I would think in most cases quick deaths may be related to poisoning like household cleanser sprays that made it into the tank but in your case fry with the weaker defense system would have all diminished before the adult.
Throwing this out there and never heard of any cases but would instant organ failure have been the cause? Can our prized Discus die of a heart attack?
I too experienced similar losses in the past which always seems to be the best and largest of the bunch.
...Ralph