Is it the stopper in the tank that is falling out or is the tank just not secure in the plane?

Tightening the stopper screw will only seal the stopper. Go too much and you could split the tank. If it won't tighten up then the nut plate on the back side is likely stripped. You can get replacement stopper assemblies at most good hobby shops.

If it is just the tank not secure in the plane then pack foam around it, glue in a balsa brace or whatever it takes to hold it in place.