If that is where the tank is going to go be very careful.
You'll be dealing with about 120 pounds of weight on the shelf.
Judging by the shelf its not ment to support that type of weight.
Check to be sure those tracks are screwed into the studs, otherwise you'll be ****** when the tank falls and you have 12 gallons of water in your office, not to mention a broken tank. :^(
Planted Tank Guru
gordonrichards2000 at yahoo.com