Water Tank Level Sensor is Leaking

It is not the metal threaded section leaking. The fitting is "spun" on to the tank and that spin on fitting is leaking. When you spin the fitting the friction of the fitting and tank heat up and that melts the tank and when you stop the plastic cools and is then welded in place. The normal fix is to replace the tank with a correct new tank. You could try to find a plastic tank welder and see if he could weld the complete fitting water tight and then install the standard well nut sending unit as shown in the other posters response. Doug