A 2000? The Cali 1100i ended in 1997. In 1998 it became an EV. Not a big deal, but the fuel level sensor did change.
It should be a 'double threaded' nut. As in, you hold the sensor from turning, and as you turn the nut, it tightens onto the sensor and the tank at the same time, compressing the gasket in the middle. Putting the nut on the sensor, then screwing them both into the tank at the same time, may not compress the gasket properly.
Putting tape on the threads may make you feel good, but it is not the right thing to do.
It is not unheard of for the tank to leak at the fitting, or for the sensor itself to leak.