I experimented on my reject guppies by giving them high fat food, they grew very fast and became fat.
(Have you seen a fat guppy ) Then one by one they just die suddenly, very lively then suddenly dead. I disected them and they all had very fat internal organs which resembled fatty liver disease.
I was also watching my prized breeder guppy male courting a female and suddenly died mid-courtship. I guess it was heart attack, he was not fat though.