When we put our house in Central Pennsylvania on the market last Summer we set up a deal with a landscaper, that if necessary, he would fill in the pond and re-seed, and as payment he would keep the fish.
Of the people who came to see the place, no one openly objected to the pind, and the couple who bought it kept the pond. Great !
It's probably a good idea to be able to tell prospective buyers that you, as the seller will pay for filling it in if necessary (really what's it going to cost compared to the selling price?). Of course, you really don't need one more thing on your "to do" list at that time, but it would mean that the pond won't be a deal breaker.
Jim McD in SVAZ