Buffered Alum is nothing more than way over priced Alum and Hydrated lime.

As Bob said, the concentration of alum must reach a high enough concentration before a noticable clearing begins. Instead of paying 10 to 20 times the cost of buying Aluminum Sulfate and Hydrated lime seperately. The dangers can be minimized by adding the lime at a 50% by weight rate of the alum (100# alum--50# H lime). That Buffered stuff is crazy expensive and is just a high profit item meant for garden ponds.

Read the "Alum kicks clay butt" thread under the Muddy Waters section and do the bucket test you'll find. Determine the pounds of alum you will need, then double it.