On large rectangular serving plate, place cake layer, rounded side down. Remove liners from 2 cupcakes, and cut them in half. Spread a small amount of frosting on cut side of each cupcake half to keep them in place. Arrange cupcakes cut sides down as shown in diagram to create back and side fins, discarding extra cupcake half. Place 2 marshmallows on front of cake to create mouth and 3 marshmallows on top of fish to create top fin. Cut remaining marshmallow in half crosswise to create eye. Use small amount of frosting to "glue" a blue candy on for pupil; set aside.