Try adding 2 square bales of good alfalfa hay to your pond. It will take a little longer to work than the alum or gypsum but is is a lot cheaper. I have used this treatment on 3 of my ponds and only have 1 that needs another treatment every now and then thanks to the bullheads.

I'll start treating my wife as good as my dog when she starts retrieving ducks.
http://geocities.com/h20fwlkillr/