SEN0257 Fluctuating voltage

My tank is 9 meters on top of the house , Sensor is connected on the ground .

When i calibrate the sensor the output is very stable.

The more water in the tank the higher the voltage fluctuation.

It fluctuates between 0.5 and 0.7 volts, so i get empty tank and then while its filling it wil jump from below 10% to 80% .

I wrote an program for arduino to take 100 samples and then show percent and send to home-assistant also show on 16x2 display.

With 100 samples tank shows full most of the time and just before  empty it will drop to showing below 10%