Templates make it easy to build this aquatic creature and the Bear, Owl, and Dog cakes too. Just cut out the parts, assemble them, and frost. When you bring one to the table, we guarantee you won't have to fish for compliments.
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 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.