In our garden there is a very large tree fern (diameter at the base about 18 inches) growing immediately adjacent to a koi pond. Over the years, the roots of the fern, in addition to being in the surrounding soil, have grown into the pond itself. At his point, there is such a large root mass in the water that the fish can no longer swim around some rock islands in the center. I would like to know how heavily I can prune the immersed roots without endangering the fern. Any information would be greatly appreciated.