Have you considered using an ultrasonic distance sensor, placed at the top of the tank, to read the height of the liquid within the tank, before and after a given process? It would be necessary for the tank to have a reasonably constant cross section .... top to bottom. Basically, ΔV=Acs×Δh.
For the cost of an ultrasonic transmitter/receiver, and a microcontroller, you might be able to accomplish your objective. ....
There is a subroutine for driving an ultrasonic range finder.
It might be in this book .... not sure, I have the first edition.:
The Quintessential PIC® Microcontroller (Computer Communications and Networks)
Amazon.com: The Quintessential PIC® Microcontroller (Computer Communications and Networks): Sid Katzen: Books