By any chance do you have a hot magnum canister filter w/ the micron filter? They do wonders clearing the really small particalsof dust thats mixed with the sand, after an initial addition to a tank. You will need to rinse it a few times, but will help alot.

I would also go to as minimal flow as possible- none would be best. Maybe just leave the overflow and return running. Do you have a filter sock? Could be helpfull. If you can adjust your return slow, alot of the dust in the water should settle in the bottom of your sump, instead of your fish tank.