If you cant find Southdown at HD, you can get 100% silica playsand that has been washed and sterilized. I bought 200 pounds of that stuff, then seeded it with 40 pounds of Aragalive. It has worked great, an did not turn brown. It actually just looks like beach sand.
I'm guessing that Quickcrete is a cheaper grade of sand, and is not 100% silica. The darker components in low grade sand will have mineral content that you may not want in you tank.
Use either Southdown or 100% silica playsand. From what I've heard, Southdown is the best.