If the tank is plastic-walled, you might try a capacitive approach... alternating parallel strips of Al or Cu tape. As the water level rises or falls, the capacitance should change. I doubt this would work on a metal tank even with insulation in between.
Another option is a differential pressure sensor. Mount one inlet to the bottom of the tank (drain valve?) and then measure the pressure. A differential model automatically compensates for any changes in local air pressure (altitude, storms, etc.).
Ultra-sonic sensors work too... but most should not become wet. Can you ensure that?
Another neat approach I saw was a resistor-ladder approach where a bunch of resistors were mounted in series and as the water shorted them out, the overall resistance changed too. (naturally at very low / safe currents).
I don't like the lights that came installed. Tank shows full until it is down to 3/4 full. It shows empty if it is below 1/4.
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