B storm,
Ground water does NOT need to be sterilized to avoid the transfer of unwanted fish. No fish fry or eggs can pass a sand aquifer. (The only exception would be something like a cavern that transmits underground stream flow to a pond.)
The neighbor's pond MIGHT be a different story.
My upslope neighbor has a waterway through his alfalfa field. It conveys water right into the waterway in my field. If your neighbor's pond were to overflow during a heavy rain event, would any fish escapees go into some type of waterway that would then feed surface water into your pond? If so, then I would suspect you would have a decent chance of getting GSF in your pond.
Another situation would be where his waterway and your waterway join together somewhere farther downslope. In that case, as the flows start to wane, fish will frequently start swimming back "upstream". I have seen fish swim up flows where their backs were out of the water! The solution for that situation is to create an "air gap" for your pond outlet pipe. Instead of having your pipe terminate in a pool of water, put a 45 degree elbow and an extension on the end so that it sticks up in the air. You would need to create a splash pad of rock or concrete so your outlet pipe heavy flows don't create an erosion problem.
Are you good friends with your neighbor? If his pond is going to drain directly into your pond, then perhaps you could help him install a better water inlet screen for his pond outlet pipe. That would be a win-win situation. His fish are more likely to stay in his pond, and more likely to stay out of your pond.
Good luck on your new pond project!
