Gypsum does not work well if at all with high hardness (> 50) . Alum will be your best bet. Your alkalinity is great for fish as 100- 200 is perfect � match pH of fish blood at 7.4. See the attached links.

https://srac.tamu.edu/index.cfm/event/getFactSheet/whichfactsheet/108/ -- Control of Clay Turbidity in Ponds


https://srac.tamu.edu/index.cfm/event/getFactSheet/whichfactsheet/112/ -- Interactions of pH, Carbon Dioxide, Alkalinity and Hardness in Fish Ponds

Good to hear from you !!!!