There is a guy who lives a few miles from me who has an interesting way of producing a "fish." He carves the body from high density foam. He applies a thin layer of drywall plaster on it, lets it harden, and then applies another thin layer. He carves/rolls the scale pattern into that layer before it dries. He installs mostly fake fins that he buys but he carves some from thin wood. He uses fake heads as purchased from taxidermy supply catalogs and paints to finish. He sells them locally and online.