First off: have you confirmed that your presumed turbity problem is due to a water-chemistry imbalance? If not, the cause of the problem could be physical (rooting fish, wave-action or run-off erosion) - and will not likely to be remedied with either alum or gypsum.
Gypsum is a safer option for correcting chemistry-related turbidity problems - though it takes more gypsum (and a longer period of time) than with alum.
