You need to find a drawing, just do a google search or get a copy of the article Anthony Calfo wrote talking about tank circulation. The tank drains over a shelf that extends along the entire back wall of the tank. If your new tank is "reef ready" it's supposed to come with an overflow in one or both corners installed, and holes in the bottom of the tank, usually 2 in each overflow area. That's what reef ready means. At least that's what it used to mean....it's an oceanic term, I think.

If your tank is drilled, you may not be able to use the Calfo style overflow, depending on where the holes are.