Good question jeo. It was a real pain in the a@#. I will tell you what I did then tell you what I would do differently.
I took out the rock then removed the cc with a cup. I then started adding the sand a bit at a time. When I realized how cloudy the water was going to get I caught all the fish and put them in a 5g bucket with water from the tank. I then proceeded to put the rest of the 200lbs of sand in the tank. I tried to put it in as carefully as I could but the tank water looked like a mud bath! It took a long time for the water to clear enough for me to put the fish back in to where I thought it was safe.
What I would do differently: Put fish in clean container with tank water (with heater and air stone). Siphon all water from the tank into a large, clean trash can. Take out the cc. Put in the sand. Slowly pump the water back into the tank. Put fish back in tank after water is allowed to clear (maybe an hour to two).
Anyway, it has been 24 hours now and the tank is still cloudy but not too bad. I think in another 24 hours it will be completely clear. Fish are doing great. I was a little worried about them last night.
Live and learn I guess.
I couldn't believe my luck in finding that sand. I bought about twice as much as I needed so if I start up another tank I will have some. <img src="graemlins//urrr.gif" border="0" alt="[urrr]" />