I have successfully used an gravity bottle ATO. I got a tunze for nanos, 3152 I think, and it works like a champ. Gravity ATO 10 bucks Tunze was 120.00
Introducing the little brother to the Tunze Osmolator, one of the industry’s most trusted automatic water top-off device and BRS Recommended. Designed specifically for smaller aquariums, roughly 50 gallons and under, the Osmolator Nano 3152 is a slimmed down version of the full sized 3155. It...
Any "nano" ATO will do, there are several good options. I've also tried the bottle ATO, which does work, but basically its only good for 1.5 days, not even a full weekend. Might as well spend the difference between the bottle ATO and a proper ATO and save yourself the hassle of refiling the bottle every 2 days.