In advance, make candy coral pieces for cake top. Pour a layer of nonpareils in pie pan. Melt white candy in parchment bag; cut bag and pipe outlines in coral shapes on top of nonpareils. Immediately pour additional nonpareils over coral, covering completely. Let set in freezer until firm. Attach coral to lollipop sticks with melted candy; let set. Repeat for each color.
For fish, prepare marshmallow mixture for cereal treats; divide mixture in thirds and tint yellow, green and orange. Prepare mini ball pan with non-stick spray; press each mixture into 2 cavities and unmold onto waxed paper. Attach ball halves together with melted candy to form bodies; press "ball" down to flatten bottom. For fins and tail, cut jelly fruit candy in half; attach to body with melted candy. For lips, cut jelly fruit candy in a V shape; attach. Attach hollow-center candies for eyes; pipe tip 3 black icing dot pupils.
Bake and cool your favorite 2-layer 9 x 13 in. cake. Position cake on foil-covered cake board. Prepare for fondant by lightly icing with buttercream.
Marbleize fondant by rolling white fondant into a ball, kneading until it is soft and pliable. Using a toothpick, add dots of blue icing color in several spots. Knead fondant slightly until color begins to blend in, creating marbleized streaks. Roll out fondant 1/8 in. thick and cover cake; smooth.
With tinted buttercream icing, pipe alternating tip 352 leaf coral and tip 16 pull-out plants on cake sides. Pipe tip 12 puff bottom border; sprinkle with jumbo nonpareils. Position fish on cake top. Insert cake top coral, trimming sticks as needed.