Get a 55 Gallon drum of yellow food coloring and under cover of night, dump it in.
Tell the neighbor that it is a toxic dump site and that you are willing to by 10 acres of his land and his part of the pond for one dollar. Be sure to use words like EPA, aquifer, contaminated, and Superfund during your discussion.
Since you will have to clean up this toxic dump site, he will think he is getting out from under a pile of expense and will gratefully sell.
During the “cleanup” you can expand the pond size and make it a beautiful place stocked with big fish.