I usually sift like 1/2 my sand bead every time I do a WC, then I do then other 1/2 in the WC after that. It would be a great idea if you did small section every time you do a WC. I can't say 1'x1' sections or anything specific. Just stick with a small area and keep sifting until you no longer see any of the brown stuff coming out of the section. It could be that you have to keep doing that same area several times until you no longer get the murkyness from the sand.