Tank Water Level Indicator Wire
The most common solutions are 

1) use a float connected to a potentiometer like a petrol/gas tank does https://www.howacarworks.com/fuel-systems/checking-and-replacing-a-fuel-sender-unit

2) use an array of binary float switches, like these, but many other brands available https://www.cynergy3.com/blog/how-select-correct-float-switch 

3) to use a circuit like yours with very small AC currents

4) use an optical sensor where the refractive index of the water causes a reflection or not. for example https://www.omega.com/green/pdf/LV170.pdf - many other brands exist

5) use an ultrasonic sensor to measure the height of water in the tank https://www.instructables.com/id/Measuring-water-level-with-ultrasonic-sensor/

6) use a pressure sensor to measure the height of water in the tank as washing machines do http://www.milanocomponenti.com/en/washing/pressure-switches-washing/washing-machine-pressure-switch/