Adding sand after you've got a tank up and running is not the greatest thing to do, or easiest, as I found out. I didn't get a whole lot of cloudy water, though. Settled within an hour or two. But you've got to make sure and clean it. I put small amounts in a bucket, then tipped the bucket and run a hose into it, stirring the sand while doing so. This stirs everything up and the light stuff floats to the top and out of the bucket, removing all the dust that comes with it. Or a lot of it.
Of course, this is all for future reference. But it might help someone else before they add sand. I also put a 1-2" layer of gravel on top of the sand, so that might have helped keep from stirring it up also. Best of luck.