PREPARE and bake cake batter in greased foil-lined 13x9-inch baking pan as directed on package. Cool completely. Invert cake onto large platter; remove pan and foil. Use a serrated knife to cut 1/2-inch-wide strip off each side of cake. Cut each strip in half lengthwise; set aside.
SPREAD cake with 1-1/2 cups of the whipped topping. Stack cake strips around outside edges of cake to make a rim. Spread rim with remaining 1/2 cup whipped topping. Refrigerate until ready to use.
STIR boiling water into dry gelatin mix in large bowl at least 2 min. until completely dissolved. Add ice cubes; stir until gelatin begins to thicken. Remove any unmelted ice. Pour thickened gelatin into center of cake as shown in photo. Refrigerate 4 hours or until gelatin is set. Decorate with remaining ingredients to resemble an aquarium. Store in refrigerator.