Double-check your water parameters and make sure you're not showing any problems there. If they're good, keep a close watch for any health issues the next few days. If you don't see any other issues, then you can probably chalk it up to incurable health issues with that fish.
We are always on the look out for potential diseases and other health issues in our tanks, which we need to be to give the best care to our fish. But sometimes a fish dies simply because it is its time to die, whether it be from internal health issues (such as cancer or organ failure), genetics (from inbreeding or a random deleterious genetic mutation) , or simply old age. We can't explain it, and it bothers us, but there isn't anything we could have done to prevent that fish's death and it's a perfectly natural part of life.
WYite
