RCI fuel cell sending unit / equus gauge not reading right

Bought an RCI 16 gal fuel cell with built in 0-90 ohms sending unit . Also bought an Equus after market gauge thats made for 0-90 ohms with 3 prongs - ground ,sending unite, 12 volt power . I installed the fuel cell like I was suppost to and followed the instructions on hooking up the Equus gauge . filled up fuel cell and the gauge read empty when full and full when empty ???? at first I thought it was a bad sending unit so I bought another 0-90 ohms fuel sending unit for the fuel cell and it still reads tge same ?? I also swapped the 2 wires around on the sending unit and still no change ???