Measuring the resistance of the water will not be a practical way to measure liquid level, there’s too many variables at play. The tank level sensor on our site is more for clean liquids, you are right, but I see no reason they wouldn’t work in a slightly dirty environment like a sump. Add a few program lines and a variable to store the state of the level sensor, watch for no changes in water level over a certain period of time and generate an alarm “Tank Sensor alarm, no fluid level change for XX days, please check the sensor”.
For an extra layer of safety you could wire one of the usual sump pump controller floats as a high level alarm, we could add them to our lineup if you’re interested.
Our analog tank sensor is a normal gas tank sensor but modified slightly to operate over a 10k ohm range which as you know, you can wire directly into the T3 controller with no external circuitry required.