Phineas and Ferb (2007)

S2E1 The Lake Nose Monster

Candace mocks her brothers for embarking on their quest to find the "Lake Nose Monster," but soon she becomes increasingly paranoid at her lifeguarding post, convinced she spots the creature lurking around every corner. At the same time, Dr. Doofenshmirtz sets out on a bizarre mission to eliminate all the zinc from the lake, even he can't quite explain his reasoning for it.