Heat oven to 350°F (325°F for dark or nonstick pan). Make and bake cake as directed on box for 13x9-inch pan. Cool 15 minutes. Run knife around side of pan to loosen cake; remove from pan to cooling rack. Cool completely, about 1 hour. Refrigerate or freeze cake 30 to 60 minutes for easier handling.
Cut cake as shown in diagram. Arrange cake pieces on tray to form fish as shown in diagram. Frost with a thin layer of frosting to seal in crumbs, attaching pieces with small amount of frosting; refrigerate or freeze 30 to 60 minutes to set frosting. Frost cake with remaining frosting.
To make purple color, in small bowl, mix 5 drops blue food color and 5 drops red food color. Drop purple color along top of fish. Drop 6 drops blue food color along center and 6 drops yellow food color along bottom. Starting from top edge of fish, blend colors into frosting with small spatula or spoon, working purple down into blue and blue down into yellow. Use back of spoon to form scales. Define lips with edge of spatula. Mark tail and fins with fork. Use candy for eye. Store loosely covered.