Well, I won't be trying your suggested test bobad...or atleast I won't anytime soon. Homedepot quit carrying sof'n soil and I can't find it anywhere else local. I'm going to go ahead and go with alum. I ordered some 'lake kleerant' which from what I understand is aluminum sulfate + sodium bicarbonate. I would rather have all my fish die than have a leak again. I'm sure your right, but I will feel more comfortable staying away from gypsum. Plus, I would never get my builder back out here to fix a leak if I do use gypsum.