I wouldn't put bullfrog outside in a pond if you happen to live in an area outside its natural range, west of the Rocky. Bullfrogs are serious pests and are eating the native frogs and amphibians. They are great escape artists, so I doubt any kind of fencing would hold them in for long. This has been tried before and bullfrog just simply climb over the fence to get to the native frog habitat inorder to eat them.