Bottom line is Alum is not for everyone, whether it be the wrong application or someone who's never done it, it can be dangerous.
I will be the first to admit that after Todd's post, I need to study the Gypsum process but getting it to the site is an issue-for me- in most cases. The difference in cost is not the only factor here but getting 40 bags of Alum vs 10 -12 ton of gypsum into a site was a challenge for me.
I'm paying 40$/50lb bag for my Alum and 8.30$/bag for hydrated lime. I charge $1,000 for app up to 1 ac and 400 per ac above the 1.
Can't speak for Rex or anyone else doing this.
My region is highly regulated on surface water due to being on the Ogallala aquafer and KS law on pond size and location, so I don't do ponds much larger than 5 ac., most are 3/4-2ac.
To this point, I have chosen my battles carefully on a pond I will or will NOT do and to be transparent, I don't treat the acreage Todd or Rex do, but my results with my choice of product have been near 100% success..
I do use both shore app with pressure nozzle and boat application.

www.aquaticspecialties.info
May God be with the Overton's Family.