When I set up my tank I rinsed the sand in freshwater by filling up a 5 gal
bucket half way and running the hose in it slowly and mixing the sand by hand from the bottom up.
I rinsed 40 lbs and then added 20lbs of new sand and had no problems.
I also filled my tank half way first then arranged my rock ( this way the water is clear) then filled the tank to 3/4 or so full, then I added my sand, then topped off. This way your rock can't shift and gives you a sturdy foundation to start from ( especially if you are planning to add gobys, or anything else that likes to burrow in the sand)
My friend who got me into SW
had about 60 lbs fall over in his 175 gal
and he is the one who advised me to do it that way and I had no problems with set up at all