South Park: Spookyfish

During her monthly visit, Aunt Flo brings Stan a fish. The fish freaks Stan out, especially when dead bodies begin piling up. Mrs. Marsh is convinced her son is responsible for the crimes and starts hiding the bodies. When Cartman exhibits behavior that is very uncharacteristic for him, the boys discover there are two Cartmans (one good and the other evil), and the gateway to another dimension is found in the local pet store.