I made 4 sheet cakes and layered it with frosting in between. Then carved the body shape and frosted it. I then made a lot of rice crispy cereal treats and shaped them into the fins of the fish. I had to shape them around bowls and cups and let them harden for a while before they would retain the correct shape.
Once they were attached (using wooden skewers) I frosted them too. Then covered the whole fish with home made marshmallow fondant that was colored orange. I then rolled out white fondant for the stripes and mouth, and applied it to the cake. I used black food color gel and painted on the black stripes, etc. I used sea shell molds to make the shells out of candy melts. The coral is made with chocolate covered pretzels that I attached together with white frosting.
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