RE: Welcome to Club SAITO !
Actually it will either dive or climb depending where the C 0f G is located. Better to consider the pump / tank pressure options. If the tank isn't installed already, and you ARE really concerned consider doing a mock up in the test stand. Position the tank in the same relative position as what it would be in the airframe and test it. Tanks make or break a good model (one component at least) but fortunately there are a large number of fixes available.
As an example when the C of G is critical in our old timers I have been known to strap the fuel tank on the side of the airframe to best determine the ideal position to achieve best glide etc.