Is the liquid in the tank grounded (or could it be), and can you stick metal foil around all or part of the plastic tank? If the answer to both is yes, you can make a capacitive sensor. You use the sensor as the capacitor in a CMOS 555 oscillator, and use the Arduino to measure the frequency. See Water level sensor w/ nichrome wire no, now capacitive - #17 by dc42 - Project Guidance - Arduino Forum and other posts in that thread.