You definitely need a flow meter. It would be far easier for you to choose one that fits the application (that is, that actually could be installed and measures the volume of liquid that passes through it) and that you can get, at a fair price.We don't know what kind of liquid you are talking about, or what the flow rate is. There is a difference between measuring 100 gallons of water per week, flowing through a 2 centimeter pipe and measuring 100000 gallons of liquid propane flowing through a 20 centimeter pipe.All flow meters operate in the same way, though, outputting a pulse after some volume of material passes through. Connect the output to an interrupt pin on an Arduino, and each time the interrupt handler is called, one unit of material has poured into the tank. The differences are what kinds of material they can handle being exposed to and what the unit of measure is.