You shouldn't leave fuel in your airplane's tank over night. The vent line to the muffler is open to the air, and over time, the fuel will absorb water though there. The carb line is also open to the air, but the needle valve restricts it down a bit. You can also get fuel syphoning out of the tank and in to the engine or muffler during handling, making a real mess. (I once syphoned a nearly full 6oz tank down to empty while attempting a repair on the plane).

That said, if you think the tank stopper is leaking fuel, how to fix it depends on the tank. If it's one of the style with a screw that goes thought he middle of the stopper, try tighening the screw. You can also sometimes seal up a tank with GOOP brand glue.

Either way, it sounds like you should remove the tank from the plane and do a pressure test to see if it's leaking somewhere.