Choose your words carefully around a six year-old

A few months ago my kids got a small aquarium with 4 fish from their aunt. Despite my warnings they struggle with the concept of only feeding them twice daily, so the fish have all gotten a bit ‘overweight’.

Last week one of the fish died so my wife flushed him. When my son discovered that one fish was missing I told him that “the fish ate himself to death”.
My son’s yaw dropped and he asked- “how could the fish eat himself?”