do you just have one layer of substrate that you mixed this into, or did you mix it into some mix of substrate, and then cap that substrate with sand/gravel?
clays tend to be made of incredibly fine particles, and will take much longer to settle out of a water column (why lake beds, mid ocean, etc. bottoms tend to form shale deposits - the only stuff that can make it out that far is fine grained clays, etc.)
If you don't have it capped, I'd suggest adding a cap. and then stuffing some floss in your filter to pull out the suspendeds.
If it's already capped, *shrugs* I dunno, try the floss/filter thing, and see if that helps?