I have a 12 year old pond 14ft in the deepest area and shallow areas through out I tried planting eel grass and American pond weed but it does not take. I planted pond lily and they have taken but something comes by in late spring and cuts the leaves off. The pond has a clay bottom with the shallows from 1 to 5ft tapering off and then a steep drop. Any one have a clue as to why plants won't grow but WLs will ?