Laser probably isn't working because the clear liquid isn't reflecting enough back to register on the sensor. Utrasonic would work because the reflection is physical and the sensor will detect the echo off the surface.
There are lots of other options too, like pressure, GWR, capacitance... although ultrasonic is probably your best bet due to being non-contact, which is preferable in the food industry.
What about a flow monitor on the input and output pipes? And/or a simple float type transducer?
Although monitoring the flow in and out works in a perfect world, it's not a direct measurement and prone to developing an error/offset. Even then you'd need two Coriolis meters or something otherwise temperature/pressure corrected and it would end up being a lot more expensive. Floats only work for point detection, which is fine for simple fill/drain controls but not actual monitoring.