Most of the time pump fed via a float or another switch, that way, as water evaporates, it automatically adds water. The holding vessel normally is a tub, bucket, etc, and then the pump or level detector is fed water from the vessel.
I'm using a ten gallon tank as my reservoir. My ro/di unit in the basement feeds the water to the ten gallon tank via a float valve to shut it off when it reaches a certain level in the reservoir. Then,I have a float switch that detects the water level in the sump. When it drops, the water from the reservoir is injected slowly into the sump via an aqua lifer pump. Once it's back up to level, the pump shuts itself and the water flow off.
I have a second float switch mounted higher, along with a water alarm, in case something sticks. It the higher float is triggered, an alarm goes off and all power to the ato(aqua lifter) pump is shut down. I also have a couple leak detector sensors connected to a module, In case water levels rise. They also set off alarms, cut off the skimmer, etc.
All of mine is controlled via an apex, but for stand alone, you can purchase ato units and leak detectors.
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