I would be polite to the neighbor and offer to pay for any improvements, if needed, on their property that would improve your pond situation. After all, the pond existed when you purchased the property. I'm assuming it existed in its present condition and there were no changes by the neighbor since then.
Pilots see a similar situation all the time. People buy homes knowing an airport is nearby. But after a few years, they start complaining of noise and demanding the airport be shut down.
