If you move the tank back, the balance will move back too. If you do nothing else, then add just enough weight forward to give the same balance point as before. Generally we don't consider the fuel weight when balancing our airplanes, they are balanced without fuel, as this would be the 'worst case' as far as the airplane is concerned. 'CG' as in Centre of Gravity' is a common, but incorrect term we modellers use when we mean 'balance point'. CG is a specific physical property, not just a convenient place to support the model for balancing.
Evan, WB #12.