One reason for muddy looking water is an electrical charge in the water that causes the water to hold the very fine particles of silt in it. I forget if it's a postivie charge, or a negative one, but the result is the water will always look muddy for all time.
In Phils pond, I believe his water was clear at one time, so it's a good assumption that it will clear up again on its own. If it was always muddy looking, then gypsum is a very good choice to nutralize the electrical charge of the water and allow the suspended silt particles to settle to the bottom of the pond.
I did this with my small pond when I built my house. I tossed all the scrap sheetrock in there all around the shoreline. The water was very, very dark, muddy brown. In about three days it was noticable clearer. Around a week later, it was beautiful!!!!! Just amazing at the diference and how quickly it happened. Then a week after that, green alge started growing. It kept growing until it was growing on top of itself to create a sludge that was building up above water line. It was the most disgusting, horrible mess imaginable. It stayed that way until winter when it died off from the cold weather. The next spring the water came back in from the rains and it turned brown again. I've decided I like brown water and will never clear up my ponds again.
Eddie