bobad,

Is there a difference in long term abiity of different coagulants to help keep a turbid pond clear? I am under the possibly false impression that gypsum lays on the bottom and helps control the suspended clay going forward. I'm not saying excess gypsum laying on the bottom will keep the pond clear, just that it adds some concentration of positively charged particles to the mix and will help you control the suspended clay with less addition required going forward than the original application. Perhaps it's just wishful thinking.

Not sure if alum works similarly or not although I suspect it does, I was too afraid of the pH swings to try alum.