[canadawest]

You shouldn't have too much trouble with the milky water and the inhabitants. Twice I've added fine grain sand to my existing fully stocked tank, and both times it was milky for several hours and none of the inhabitants seemed any worse for wear.

The fish tended to hide in the corners, or where there was circulation during the "snowstorm" but otherwise were ok.

I recommend turning off most of your powerheads to allow the finer silt to settle rather than circulate around. Also it helps to keep your skimmer running during the milky phase, as the finest grains will be removed by the foam and the tank clears up much quicker.

After a couple days the sand will get a bacterial coating to weigh it down, and it won't blow around as easily anymore.