SEN0204 level sensor doesn't like my tank.

I want to use a SEN0204 to sense when my water tank is nearly full. But the sensor can't cope with the wall thickness of my tank. The tank is made of polycarbonate. Without damaging it, I can't measure the wall thickness exactly, but I'm pretty sure it's half an inch or less. I've tried working at 5V and 13V.

Any suggestions please?

TonyMS