My sediment pond is approximately 20,000 gallons. It is terribly muddy, like coffee with creamer. I don't want to use alum and quicklime if I can avoid it. How much gypsum would I need? How to spread the gypsum? Wet or dry?

Bump.
I put a pinch of gypsum in a half gallon container of the sediment pond water yesterday, shook it up to dissolve. No clearing overnight. Today, I put a pinch of alum in. The water cleared within an hour. Maybe gypsum is ineffective on some ponds, IDK. Any thoughts on this?

John, do you have fish in it?

Yes, some minnows, and lots of crawdads.

Would they be much of a lost to you, if you used the alum and they didn't make it?

It's the only sustained minnow population I have right now, so I need them to survive. They have been living in muddy water for over a year, since I pumped out, killed, and restocked the sediment pond.

gypsum doesn't work on my pond water in bucket tests. I'm about to grit my teeth and drop a bunch of money on alum. Hydrated lime is used to prevent the pH drop of alum from killing fish. Try the alum bucket test again with minnows in it. mine survived even without the lime.

Gypsum did not work on the jar test, but a pinch of alum powder did, and cleared the jug of water in less than an hour. I only have a very small jar of medicinal alum. I need about 20 pounds of it to clear my sediment pond. Where could I get 20 pounds of powdered alum at a reasonable price? I aim to mix it with 10 pounds of quicklime in a tub and apply the slurry with a hose with a nozzle connected to a small water pump.

Some Co-ops have alum or can get it.

Our farmers co-op network around here cannot get alum except in the little 4 pound flower bed bags, and that's very expensive per pound.