Heathcliff only tries to eat the goldfish on Monday. despite there being no humans in sight and no possible escape for the fish, the same fish keep surviving to see the same ordeal every week. Why? Because they're mutants. Seriously. If they can remember things that happened on Monday, last week, they'd freaking have to be. Soon they'll have legs and lungs and sharp teeth, then it'll be Heathcliff who hates Mondays, and vengeance will be complete.