esshup: Yes, the diffusers (two 9" micro bubblers) are attached to a mount that gets them about 10" off the bottom. When we first got it going earlier this spring, we noticed the increased turbidity problem, so I hauled it back up and mounted the whole shebang to two bread trays that got it another 8" off the bottom. So I'm figuring it's far enough off the bottom not to be the cause of the turbidity. My best guess is that there's so much suspended clay in the water column (about 12-13' where the diffuser is located) that the diffuser is just churning it around. We also have a bunch of koi in the pond that might be churning it up as well.
I'll be adding the alum today. We figured we have about 0.5 acre feet so judging from the tables we've found here, we're going to add about 25-30 pounds of alum.
