Ask a decorator to make a fondant or gum paste angelfish, clownfish, or butterflyfish, and position it along with seahorses, starfish, sand dollars, sea anemones, or shells.
Fish costumes have remained a Halloweens staple for children ever since 2003's Finding Nemo, a Disney move that features a lovable clownfish.
Costumzee features a costume that will allow your child to make believe he is an orange clownfish, just as Nemo himself is.
A Burda costume pattern is linked here that makes either a clownfish or a shark, but the costume can easily be altered into another type of fish altogether if you wish.