You don't have a clam by chance do you. Mine decieded to spawn over the weekend, and it got so milky in the tank at one point I couldn't even see two inches into the water. If not, the odds are that you are having one of three things happening. First and most likey based on your coments is that you are having calcium carbonate percipiate out of the water. This can happen if you have fast pH or alk changes. It's always best to match both cal. and alk. of water change water to that of the current tank parameter prior to doing any water changes. Another possiblity is that the rearagement of the rock work and such is causing someplace in the sandbed to be continually disturbed..and should be fairly obvious to find. The last one, and I'm not to sure on this one, is some kind of bacterial bloom...and I would think you would need either high ammonia or nitrate/nitrite to fule it.