If you can figure out how to have less evaporation your problem will be solved. :wink: Also, make sure your ATO water line isn't under water, it will create a suction and cause water to drain back in your ATO container. That would cause your ATO pump to kick on a lot.
Yes, either use two float switches with a set distance between ON and OFF to open and close a solenoid valve or a digital sensor such as the Spectrapure UPLC-II uses which can be easily adjusted anywhere between 1/10" and 4" differential at the touch of a button. One switch or a float valve will never be efficient and will lead to TDS creep issues and short DI life if you feed it directly from the RO/DI.