Wrap a rimmed baking sheet with plastic, pulling wrap taut over top of sheet. Scoop desired flavors of sherbet using a 2 3/4-inch ice cream scoop, gently dropping them onto the plastic wrap as you work. (Scoops should not touch the metal; this will help them keep their round shape.) Freeze until set, about 30 minutes.
Meanwhile, make decorations: Stand jelly candies upright on a cutting board. Cut candies lengthwise (vertically) into 3 thin slices. Cut outer slices in half for the fins and tails. (You should have 6 halves of the same color for each fish.) Cut long, thin strips from middle slices (12 strips total); pinch ends together to form circles for the mouths. Using the blunt end of a skewer, dot melted chocolate onto the white candies for eyes; let stand until set.
Cut cake into 3 layers. Cut out 4 rounds from each using a 1 1/2-inch round cookie cutter. Place each in the bottom of a double layer of paper muffin liners.
Working quickly, place 1 scoop of sherbet on top of cake in liner. Gently place 2 jelly pieces on sides of scoop for fins, 3 in the back for a tail, and 1 on top for fin. Add candy eyes. Return fish to freezer. Repeat with remaining scoops,working with 1 at a time. Freeze until set, 1 to 5 hours, before serving.