I have recently moved the location of my tank from one side of the room to another. The tank was emptied & the plants removed but the substrate was retained however I added a new top later of soil & sand. Every thing was going fine until I added 5 corydoras - they are stirring up digging at the sand to the extent of clouding the whole tank, It looks like the tank is full of suspended matter. I think the top layer of sand is to shallow & they are finding the under soil. How can I add more sand without emptying the tank again?