Fuel gauge working opposite

[2001convertable]

Hi, I installed the fuel tank back in the cab today. I took the wire off the rear tank last week and ran it into the cab for the relocation back into the cab. I figured that I would check the sending unit with my multimeter on resistance.With the leads red on sender terminal and black onto housing. As I raised the float to full max the meter read 30 ohms which is good. I grounded the wire to the old tank that I routed back into the cab and the gauge went from empty to full.I then took the sender unit and attached the fuel gauge wire to the connector on the sender and grounded the housing. With the float down the gauge read full. When I raised the float it went to empty. The sender was grounded as well. This is working oppositely. I was wondering that the rear tank install from previous owner was wired for that tank and there would be some type of resistor of capacitor or something or maybe the gauge has the opposite polarity. I also felt a little electric charge as I was holding the sending unit.I took a voltage reading at the hot wire going to the sender and it read 7.42 volts. I read the battery in the truck and it read 12 volts.If someone has an idea please help....thank you