Oh! This sounds a lot like the problems I have had for the past year. Like @zboblamont I have been pulling my hair trying to find a solution.
The tank is a square profile septic tank, cross section 1 x 1 m. Access to the tank is through a manhole some 60 cm diameter.
First i tried mounting the sensor in the lid of the manhole but all the time I ended up with measurements like 20 - 30 cm, no matter what the liquid level was.
I then tried to mount a baffle around the sensor trying to limit the beam spread so that the sensor would not see the corrugated wall of the manhole. No luck!
Next idea was to increase the powerfeed to the sensor. First it was powered from one of the digital pins of the Arduino, giving some 4.x volts. So, thinking this was below specs, I built a circuit that gave the sensor proper 5 V as needed. No luck!
The final solution was to mount the sensor at the bottom end of a plastic tube, 30 mm outer dia and then feed the cable through the tube and out to the electronics. The tube is attached to the wall of the manhole using a plastic clip. Please see attached photos. Success!
So clearly my problem was caused by the sensor getting reflexes from the corrugated wall of the manhole.