Yup, ATO = Auto top Off. It's a switch (or switches) that controls a relay to turn a pump on and off based on the water level. So when water evaporates, the float switch drops, closing the relay and allowing a small pump to pump water into the tank from a holding bucket.
I ordered the dual-switch design from autotopoff.com. I've heard nothing but good things about them, so I wanted to give them a try. I ordered on Friday evening, and received it on Monday. I hope to get it hooked up tonight.
In the DC Metro Area? Check out GWAPA