I am with the Commish - DONT DO IT! The little pieces of fiber glass are going to be in your pond forever. I do not claim to know what it will do to your fish, but you are what you swim in... Home Depot sells Gypsum in 40 lb. bags for $4 - you will also want to invest in one of those dust masks from the paint department. Kids do pretty good with a Big Gulp cup and a couple of bags each for a little bit of "white-washing-the-fence-kind-of-fun" race on emptying the bags.

We put in 800 lbs and then another 400 lbs for the suspended material to drop. It would take a whole flat bed truck of plank to get it done...