turn off your closed loop if you have one, as long as that is running, the sand that is suspended in the water column wont have the ability to settle. Also, look at where are your sump returns pointing too, if you did as many have done here and oversized the return from the sump, meaning that you have more than 8x turnover per hour, this will also not allow your sand to settle, so try and restrict the flow a bit. right now you dont need that much flow. just my 2 cents