South Park Episode 215 - "Spookyfish"
Sharon Marsh's Aunt Flo makes her monthly visit and she has present for Stan, a fish.
Stan's fish really freaks him out, especially when the dead bodies start piling up. Mrs. Marsh is convinced Stan is doing the killings and begins hiding the bodies. The boys discover there are two Cartman's and a gateway to a parallel universe at the "Indian Burial Ground Pet Store."