http://wildlife.tamu.edu/publications/taexponds/0905a.pdf Try this link and run the slurry tests for the correct amount of gypsum and alum to use. Also, need to get test run for alkalinity and pH of pond, soil office should be able to help with this or buy the test kits and do it yourself. Alkalinity should be above 20 mg/L. If not need to apply lime. Alkalinity above 50 mg/L gypsum won't be effective.
With high rates of alum and low alkalinity (below 20 mg/L) Hydrated lime must be used with the alum to reduce the acidifying effects of alum. The rates of alum usually run 150/250 lbs /acre. Without hydrated lime, the acidifying effects could cause low survivability of your fish.
