The Grim Adventures Of Billy and Mandy (2001)

To raise money to buy Sea-Critters (similar to Sea-Monkeys), Billy has a garage sale where he sells Grim's scythe to Irwin for less than two cents. With the great mystical strength and power of the scythe, Irwin hypnotizes Mandy and takes her on a romantic date. Billy and Grim try to recover the scythe, and when they do Mandy gets out of the trance. Irwin is punished by being transformed into a tiny sea-critter and placed in Billy's fish bowl.