I had mysterious deaths of a couple fish,shortly after feeding.
Healthy active wrasses eating & swimming normal, dead w/ in hours of feeding.( Potters wrasse I had for over a year & a radiant wrasse I had for several months) they were aggressive feeders & would eat more than most of the other fish.
Only thing I can think of it being was Vibrio,or some other bacteria/Pathogen in the food.:(
I tossed the batch of frozen foods( blood worms, Spirilina (?)Brine shrimp, fish eggs & LRS) .
Knock on wood no losses like that since .
I still blame the food,even though It wasn't left out or contaminated as far as I know