PREPARE cake batter and bake in greased foil-lined 13x9-inch pan as directed on package. Cool completely. Invert cake onto large platter; remove pan. Use serrated knife to cut 1/2-inch-wide strip off each side of cake. Cut each strip lengthwise in half; set aside.
SPREAD cake with 1-1/2 cups Cool Whip. Stack cake strips around edges of cake to make rim; frost rim with remaining Cool Whip. Refrigerate until ready to use.
ADD boiling water to dry jelly powder in large bowl; stir until completely dissolved. Add ice cubes; stir until jelly begins to thicken. Remove any unmelted ice. Pour thickened jelly into centre of cake. Refrigerate 4 hours or until jelly is set. Decorate with remaining ingredients to resemble aquarium. Keep refrigerated.