I doubt you'll have to add weight, but that may depend on where you mount the tank. My tank is mounted just ahead of the cg so it leaves the front of the plane, behind the firewall, open to move the receiver battery up there and balance. With my YS63 and the Rx battery up ahead of the wing saddle bulkhead it balanced at the 5.5inch back from the LE as recommended as a starting point. After some more flights I'm going to move the battery back until it's under the tank (as viewed if the plane is upside down). That should move the balance point back to 6.5 inch from LE for better 3D performance.

Mike