A Fish Tale(2000)

Two boys named "Chuck" and "Fly" go to the seaside and take Chuck's little sister "Stella" there with them. They randomly stumble upon a distracted scientist's lab. Stella drinks a potion made by a scientist and turns into a starfish. Fly and Chuck also drink the potion to go after him. The scientist tells them that if they do not drink the antidote within 48 hours, they will remain fish forever...