Ruby Red Death - please help diagnose

[calico.100mph]

I just had my Ruby Red die. No real changes in the tank. I did a 40% pwc on Sun. He was eating fine then just vanished. He was 3 5/8 in long and coloring beautifully. My Jake pulled him from his burial site so I pulled him up. After inspection he had no signs of real trauma. Stomach was normal not distended. The only routine change was I fed frozen that night as a treat instead of flake. brine shrimp, blood worm and some veggie matter.
Has anyone experenced this type of death. It seems I have done this now for the fourth time with different fish.

Ruby Red
Stepens eye
Bi-color
Malari

All dead the same way about 2 months apart.
I have 18 other healthy fish in that tank.

PLEASE Help ME!!!

 

[HUKIT]

I would start with basics like what are your current water parameters? Could have been a large swing in ph? Any other signs of illness like stringy white pooh or reclusive behavior?