Steve - I had the same problem with my pond. It is probably suspended clay. Let other experts weigh in here - I am a novice...however I learned through the forum that it has to do with positive and negatively charged particles that keep the clay suspended. I had my water tested and then added a couple tons of low cost Ag lime from the local farm coop on our area. Within a few weeks the water was clear after being muddy for 1 1/2 years. I also added gypsum - but not sure that was neccessary. I had tried old hay and other recommended home remedies, but the lime did the trick.

"Our Life is frittered away by detail. Simplify, Simplify" -Henry David Thoreau -