Dekalb08,
The problem with alum is that it can cause the pH of the water to drop (become more acidic). Drastic pH swings can kill fish. The question is whether or not it will occur in your water. Where I am located (NW Missouri) most of the ponds are well buffered and are resilient to the huge pH shifts that can occur with adding any acidic substance.
Folks in the southern part of the state aren't so furtunate so they often have to add AG lime with the alum. The ag lime helps balance the pH or counteract the drop in pH by raising it. Think of it as an equalizer.
I beleive there are certain areas in Texas that often require the use of Ag lime with alum, but other areas don't need the ag lime. Not knowing where these regions are or the dynamics of your soil, I suggest you contact a professional biologist or soil scientist in the area that is familiar with your area. There are several Texans that frequent this site that could help you including Bob Lusk and Todd Overton. Or perhaps someone else on the forum lives close and can communicate their experiences.
I've only messed with alum a few times, and the amount needed to get the results we were after always exceeded what was recommended by various websites, extension publications, etc.
Good luck and keep us posted. Shawn
