Sheetrock is gypsum. Read below...
http://www.conservation.state.mo.us/manag/clearpon/

JeffC wrote:

Hi - my neighbor has a 1 acre stock pond that get's covered in Algae all
summer long. I've heard that throwing cut up Sheetrock help dissapate it.
He's all for it but neither of us know what damage we might do. There are
currently no fish in it but a lot of frogs and turtles.

Thanks,
Jeff