The only way you could make this work with buttercream would be to draw on white edible image icing sheets with edible markers and cut them to fit your cake,and wrap your cake with them and cut one in a circle for the top.
I have done several fish cakes and I always take a small paint brush and paint the eyeball with piping gel. Works great but works best if done at the last possible minute so it retains the wet look. Once it dries it is not so shiny like a fish eye should be.
Here is a link to the complete set. Cutter, veiner, and center mold. I have this one and it is very time saving to be able to mold the center.
Their hydrangea leaf cutter and veiner is also awesome.