Todd Overton's fish farm in TX sells a ton of gypsum for $160 or .08 cents per lb. Alum is $50 per 50 lb. bag, $1 per lb. So, alum is actually 12.5 times more expensive here. I have used both and for my water chemistry, gypsum has a lot more bang for the buck. It also eliminates any PH/fish kill issues. You can also notice subtle differences in water clarity when spreading smaller applications. A simple jar test using both alum and gypsum would be a good starting point to see what works. There is no dispute that Alum works almost immediately and is most effective.