Read this link

https://srac.tamu.edu/index.cfm/event/getFactSheet/whichfactsheet/108/

Note - your water has Ca hardness over 50.

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.

Last edited by ewest; 05/11/12 08:07 PM.