Templates make it easy to build this aquatic creature. Just cut out the parts, assemble them, and frost. When you bring this cake to the table, we guarantee you won't have to fish for compliments.
Place templates on sheet cakes, and cut out with a paring knife. Discard scraps. Halve smallest fin piece and round eyeball piece horizontally, discarding bottom halves.
Arrange pieces into a fish shape.
Starting with 6 cups buttercream, reserve 1/2 cup and tint the rest pale blue with food coloring. Use blue buttercream to attach fins, tail tip, and eyeballs to body. Coat eyeballs with untinted buttercream and the rest of cake with blue buttercream, rounding over the fin in the center of the body. Refrigerate until frosting is firm, about 30 minutes and up to overnight. Add a second smooth coat of untinted buttercream to eyes. Layer more blue buttercream on fins and tail, creating grooves with a small offset spatula. Add more food coloring to remaining buttercream to darken it. Place in a pastry bag fitted with a small curved petal tip (Ateco #61). Starting at the edge of tail, pipe scallops across the body, overlapping each scallop and each row.
Use sunflower seeds as pupils and gummy candy as lips.