Calcium Carbonate rock or Tufa

[Canadian Man]

If you go to petstores you will pay too much. It is availiable at pretty much any petstore btw.

Where I got it from was a friend who got it from a rock type supply company. I am sure you could find something in the yellowpages.
He got a box full for very cheep. I ended up paying about $0.45 per lb from my friend.

If you look at this link the piece of rock in the center of Alan's tank is a piece of calcium carbonate rock that used to be in my tank. You can see the part that was burried in the sand by the white color and the rest of it looks great!
http://members.shaw.ca/reefs/AJ_77/33gal/index.html