I have started maintaining a large pond at an elementary school. The pond has installed over 15 years ago and was not cleaned untile I took over the mainatnance. I emptied the pond and removed all sediment that I could, but there is still a lot of material in the filter medium in the bottom of the waterfall. The bottom of the water fall appears to have 2 layers of a fiberous mat and a layer of what appears to be pourus rocks. I can not get the muck out with the materails in place. Should I remove all the materials and cl;eam and reinstall, or is there and easier way?