Spensar, I need alum for the reason Ben mentions: Lots of dead algae. For seven months of the year I don't use the pool and I cannot afford to keep it chlorinated on my limited income. Plus, I get LOTS of plant and even some animal biomatter in the water, just the location. There's no way to filter this mass of slime and leaves, etc. So, alum to the rescue.

I use four or five cups broadcast over the pool and in about two days it's ready to scoop out the big stuff and then vacuum to discharge for the dead algae and dirt.