Clarifier and flocculent can work, but they can also be a royal pain. Clarifier seems to work better than floc. The issue with floc is that it's meant to settle particles in the water to the bottom, and to do that the water has to be still. The issue with that is that as soon as you start the pump to vacuum, it's not still anymore and the settlement gets stirred up so then you have to repeat the process of letting it settle, then vacuum, and it invariably takes several rotations of settle then vacuum to get most of it out.
P.S. It may be too late now, but except for a very few specific situations, the sand never needs changing.