How far from the top? How thick is your glass? And finally, how tall is your tank?
(Physics major mode engaging)
Remember, at the bottom, there is a lot of water pressing out on the glass. If your glass is only 3/8 inch thick and you have a 1/8 inch crack, you have only 66% of the material left in that weak spot. Think about the mechanics of it, and although gluing the piece back may make it look nicer (or not), there is still a serious defect in the material that gluing doesn't repair 100%. If it were my tank, and the scratch was at the bottom, I'd seriously consider replacing it. If it were at the top, I'd probably keep it. That's just my $.02 though.