I have the two jars of pond water. They are probably 1/2 gal and I put 2 teaspoons of alum (from my spice cabinet) in one and stirred it up. Will leave it for several days. I'm not sure if the two alums are the same. If not, please let me know and I will get the right alum.
Ann
p.s. I read the link sent and saw the pictures. The second picture looks very much like my pond water.