How does your pond get water? Is there any stream near your overflow?
I'd suggest a treble hook baited with some chicken liver to see is some Bullhead Catfish found a way into your pond. Also do a Jar test to see if anything settles out of the water. Another possibility is that you do not have clay suspended, but instead have a brown algae growing. Suspended clay looks more like heavily creamed coffee, where a brown algae is a considerably darker, more like a chocolate milk.
