PA- bought property, had it surveyed , found out neighbors in-ground pond is 15' over. ?

Contact your real estate lawyer, unless the neighbor has a registered easement, you now own 15 feet of water. The question is, how much is it worth to be on bad terms with the neighbors? If it was me, I would either just make a point of mowing my grass to the exact edge of my property, or put up a fence that leaves a gap where the pond is.

Send them a certified, return receipt letter pointing out that the pond crosses onto your property. You can either give them permission to continue using it or ask that they remove the pond from your property. Depends on how much you dislike the pond and or care about living in peace with the neighbors.

There's a lot more to adverse possession than just using the property for a few years. Giving them permission will prevent them from every being able to claim the land

unfortunately for you, the law says if a property line is encroached upon and the owner does not file objection within first 5 years, the property line "moves" to accomodate... the previous owner gave away your property

should have had survey before buying, since you didn't , not much to be done here, you can go back and try to get previous owner to compensate you for his error, but good luck with that

as for filling in some, be careful, might be better to have good friendly neighbors and deal with it than to piss them off permanently