The circuit shown uses relay logic with a mechanical float switch.  No electronics is used.  The circuit shown operates using 24vac parts but the same concept will work with other AC or DC voltages.

Pressing the “fill” pushbutton switch latches a relay on.  The latched relay contacts close and routes power to a solenoid valve.  When activated, the solenoid opens a water valve.  With the value open, water begins flowing into the tank.  Water flows until the water level activates a normally closed float switch.  When the float switch opens, the relay is unlatched, turning off the solenoid valve and the flow of water.  An indicator light turns green when the valve is open and turns off when the tank is full.  Once started, the operator can walk away, allowing the circuit to automatically fill the tank with water.