Your decision on whether to use alum or gypsum should be based on experimental simulations in sample jugs. For gypsum simulation, I use 1.4gram additions of gypsum to a 1 gallon sample in order to simulate 1000lbs per acre/ft of pond water. For alum simulation, I use .14gram to simulate 100lbs per acre/ft.
If you use Alum, you also need to use hydrated lime at a rate that corresponds to 40% of the total alum used. Also you definitely need a dust mask!
I lean towards gypsum for most cases, because raising calcium hardness is beneficial for overall pond health, bulk amounts of undissolved calcium may stay on pond bottom for available dissolution in the future, and there is less risk of harming the fish.
Easiest way to disperse gypsum in small pond is to dump by pond edge, on a slope, then blast down the pile with a water pump and jet out the slurry with a boat motor. Just dock the boat against the shoreline and propel the slurry out.
We supply bulk gypsum at $150 per ton here at our live fish market. Your 1800lb gypsum quota won't take long to disperse, using the method I described. My advise is to double or triple the gypsum amount if you are going to all of this trouble, because you want this to work, and for a long time!
