There are a lot of nice marginal/emergent plants that will soak up nutrients and help keep the pond healthy. There are also some very nice and well behaved water lilies available. I have had poor luck establishing Vallisneria (eelgrass), but this appears to be a good submerged plant. What is the maximum depth of your pond, and how much of your pond is that deep?
![](http://i914.photobucket.com/albums/ac341/raherman/Duck%20pond_zpsbmloqr2y.jpg)