Dr Andy, you may already have done this but if not, then get some of the reeds and grasses that line the streams and lakes nearby and plant them around the lake perimiter. You can even use rice pants, just let them go to seed and remain in the ground. They will bind the soil and prevent erosion. If you plant lots more lilies, hyacinth, lotus and other floating aquatic plants your water will probably eventually go clear. Try to avoid the spirella type plants which will fill the pond up and prevent other plants from flousihing. The taller reeds look great and will help to shield the pond from predatory birds which might take your fish!