A good designed pond with great filtration shouldn't need a water change. I have never conducted a manual water change on any of my ponds.
Of course if we have plenty of rain and the pond overflows this is a water change. A pond will benefit from a water change but if we can create a well balanced ecosystem it shouldn't be necessary for us to do it ourselves.
Let mother nature take care of the pond is my motto. The key is having a good filter and plants. My preferred filter is always a bog or wetland filter.
