Is the diffuser set right on the bottom of the pond? Is there any sort of dirt screen under the diffuser? If it is, and there isn't, I'd pull it up and either make a stand to set it up off the bottom of the pond by a foot or so, or place it in the center of a 2'x 2' or 3'x 3' piece of plastic to help stop it from stirring up the bottom sediment.