Posting this partly because I’m not sure I ever saw the original, unedited 1969 version, on television, and partly because Mark Evanier is here to give you the full background:
The fox is Paul Frees, who was the voice of everything while he was alive. The turtle is Ralph James, an actor who is probably best known today for voicing Orson, the commander of Mork on Mork & Mindy.
He runs down all the voices. Check out his post here.