Jim's suggestion is right on! If afterwards you have any really bad spots, try full strength Muratic Acid, but be VERY careful handling or breathing the fumes. Best to use ruber gloves, and be in a well ventelated (outside if possible) area. Rinse it out REALLY REALLY good - any residue will really mess up your PH.

Muratic acid can be purchased at most pool supply shops, and usualy at Home Depot/Lowe's.