This is a must read fact sheet on the subject. If you read this fact sheet and then need more info then ask again and we can go over the fertilizer method you tried.

I assume you do not have muddy water run-off. If your well water does not have a clay problem and you got it under control by the fertilizer method then my guess is the aeration is causing the problem by re-suspending the clay solids after they drop out of suspension. You may need to elevate the diffuser off the bottom some. If you try either alum or gyp. and it works and then the turbidity reappears it is the aerator.

SRAC 460 Control of Clay Turbidity in Ponds

http://srac.tamu.edu/tmppdfs/13452410-46...d1522b32031395e

From SRAC Water Quality Fact Sheets

http://srac.tamu.edu/index.cfm?catid=25