It is normal for a model engine running on suction or even muffler pressure to lean as the level in the fuel tank goes down. That's because there's less fuel above the pickup, and so less weight pushing the fuel out.

You want your fuel tank positioned so that its centerline is even with the center of the caruretor or slightly below it. That's so you lessen the leaning that happens as fuel is consumed. The farther the tank is from the engine, the more pronounced this effect will be. More than a few inches, though, and you'll need a fuel pump or a pressureized/regulated system.

With the fuel tank located properly, you lean the engine at full throttle so that it's running rich of peak RPM. Do this with the fuel tank about 1/4-1/3 full and with the nose of the airplane pointing straight up. Now fill the tank and fly. It will be richer than you just set it, but don't lean it more. You don't want the engine to go lean of peak RPM when the fuel level gets low.