Gas tank gauge went haywire when gas tank was replaced?

Ever since the gas tank on my 96 Ford Ranger was replaced with a used gas tank, the gas gauge has been inaccurate. I've noticed the dial drifts up and down when it gets to about half a tank full. The other day I ran out of gas on the highway with the dial slightly over quarter tank.
The tow truck driver said that it's probably a used tank from different kind of Ranger so the sensor needs adjusting OR the small floatation sensor inside the tank is messed up. I'm taking it in next week back to the place that installed the tank.