Bearded dragons by no means need a 120 gal or 6x3x3 which is over 400 gallons. That is ridiculous for one lizard. The smallest recommended is a 40 gallon breeder, or 55. That is fine. They really will not utilize 120, gallons or 400?!? If you really want to i guess you could, but a smaller tank is fine. Once they are full grown, they really dont move around much other than to the basking spot and to eat, and to get out of the basking spot. I have 2 at the moment and have been keeping them for 7 years.