Back to the fuel gauge

Okay. Origionally I started with a problem in the fuel gauge. I pulled the fuel tank to get at the fuel sending unit. Now that the "NEW" tank is installed I'm back to square one!

After removing the tank and pulling out the fuel sending unit, I connected it to it's jack, and using an alligator clip, grounded it to the ground strap that runs from the fuel line to the sending unit inlet. When I moved the float by hand, the gauge worked fine. I reinstalled the tank in the car, put 6 gallons in, and.......same problem once again. When I turn the key on, the fuel gauge registers just over empty 1/8 to 1/16 full.

Does anyone have any suggestions so that I can fix this problem?

Thanks in advance!!!