Capacitance will vary from external influences. Come up to a traffic light with the magnetic sensors buried in the road and see what happens. Or put a big steel truck nearby.
If you tap the bottom of the tank you should get a return echo and maybe (depending on the shape of the tank) it will have a frequency characteristic of how full the tank is, same way you can tune water glasses with different amounts of water/wine and play music on them. Perhaps 2 piezos to make a tap/echo system?