yeah you can.. although most piranha don't look good from the top.. you wouldn't be able to see them in all there glory.. theres nothing like seeing the light shine down the flanks of a huge piranha.. and you just wont get that in a pond.
you don't have an inquisitive cat or dog do you? lol think an aquarium would be best to view them BUT a pond would be cool as you could see the feeding frenzy from a new perspective