so I backed up and decided to use a little science (like I should've done in the first place). concentrations of alum are really small and I don't have anything that has good weight sensitivity in tenths of ounces (what I'd need unless I had a really big pond sample). so did it by making a solution and adding teaspoons at a time of the solution to a gallon of pond water. without going into the calcs, one more 50lb bag doesn't do anything but 2 more and everything drops out in a couple hours. Past that no change.

tells me two things (that I expect you all already know)
1) there is a magic concentration. Until you hit it, nothing much happens
2) you can build the concentration up (ie don't have to add all the alum at once)

so ordered more alum. fingers crossed