I have created one circuit which consists of 2 BC transistors low rating any values, one is NPN and other one is PNP, Relay and battery or battery eliminator with 1.5 to 3V.
In this circuit PNP and NPN transistors together to form SCR gate or we can use SCR gate instead of two transistors. Gate wire of this circuit is at the top level of the tank. Other two line wires are at the bottom of the tank. Function: Initially over head tank will be filled, so wires also connected to each other by water. Water will complete the circuit and so, relay will be off in 'F' end, so motor will be off. Even after the water level drops down from 'Level A' the circuit will be 'on', so Relay will be 'off' at the 'F' end. When water level drops down below the 'Level B', circuit will be (open) 'off', so Relay becomes 'on' at 'F' end and water pump will turn 'ON'. When water level rises it will touches the two wires but still the circuit will be (open) 'off' condition. so Relay will be (on) at 'F' end and motor will be 'ON' to fill the water. When water level reaches 'Level B', all the wires will be touched by water and forms (closed) circuit. And Relay will be (off) at F end, this makes motor turn off.
I have tested this experiment and succeeded.
I will upload the image and video in coming days. Currently, I'm busy with another project. However, you can do this project by referring the image.