I recently finished construction on my pond and its got about 6 ft of water. I went swimming in it yesterday to see how deep it was and theres about 1 ft to 2 ft of gunk above the bottom of the pond. Its not thick or anything but real mushy feeling. About 90% of my pond is clay. And also since i have so much clay i put a bunch of drywall in the pond to clear it up ( which worked great) but i wonder if all that gypsum is what i felt or if thats just how the clay bottom is? Does anyone have an idea of what that gunk is and is it habitable or harmful for fish?

Build it and they will come, whether you want them to or not.