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Add about 1/2" of sand at a time - this will give the existing critters in your LS a chance to 'migrate' up to the surface without being completely buried.
I'll just add that lots of folks suggest using a piece of PVC with one end near the bottom of the tank and pouring your new sand down through the PVC to reduce the amount of silt that gets stirred up when doing this. But it will still get cloudy. It won't hurt anything and should clear up in a day or two (depending on how cloudy it got to begin with)