In the neighbourhood of the tank (if it is made of plastic or wood) you may insert steel balls and, using a bar tracker, you may detect the position of the balls inside. If you don't want to use the bartracker, just connect a coil to a multimeter and every time the impedance peaks, you have the position of the balls.

An anlternative method is to install tension gauges (like the ones used for computing rock strain in comtression test) and assess what happens to your piles.