Read this and note in particular the quoted part below. So find out your alkalinity first.

http://srac.tamu.edu/tmppdfs/13452410-46...a25e1fc69a57d1b

Although not nearly as effective as alum, gypsum also can be used to control turbidity but without the loss of alkalinity. Gypsum must be added to achieve a concentration of 100 to 300 mg/L for effective turbidity control. For most ponds, gypsum application rates will range from about 1,000 to 2,000 pounds per acre (Fig. 4). In hard-water ponds (calcium hardness greater than 50 mg/L), the water is nearly saturated with calcium and gypsum may be ineffective. In that situation, alum will be the only effective coagulant.