A solenoid valve would be operated by a water level sensor or a float switch. Similarly, the float switch is in the top of the tank, and it opens the valve when the level drops, closes it when the float comes up. You can reverse the action using a relay (if the switch or the valve are the wrong way round). However, the solenoid valve should be closed with no power, if you don't want the tank to overflow for a power failure. Similarly the relay should operate when the tank level drops, so that the solenoid valve opens then. The float switch contact closes when the float drops. The relay contacts close when it is operated, and the valve opens when it is powered. The float switch will have hysteresis. That means when the level drops enough to operate the switch, the level has to increase past that point before it switches off. This stops the valve cycling with small changes in level. The other purpose of the relay is to relay the switch contact on the float. The float switches 12 or 24V AC or DC to suit the relay coil. The solenoid valve is switched by the relay contacts. It could be the same voltage supply if that suits the solenoid valve, but a higher current than the contact rating of the float switch.

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