After maintaining aquariums for over fifty years, I've found replacing evaporated water always one of their most necessary and regular maintenance tasks. Everything from simple drip lines with IV type controls to very expensive computerized equipment, some that were even used in hospitals, have been tried to make that chore less time consuming. Did I find a device that was 100% dependable and needed no follow-up except for keeping the water reservoir full? Not really, but close, yet they were quite expensive and needing fine tuning as to the daily quantity dosed because of seasonal changes in evaporation rates. And besides, they were sensitive to power failures, as their capability to automatically reset after a power loss did not always occur.
As for float type ATO devices, I've tried many. All had to be (or should be) placed in a snail-free zone where water movement is not overly swift, yet has enough movement to prevent surface scum from forming. And also preferably, a dark area where algae would not flourish and interfere with the float. That usually meant it was better to place it in a sump where environmental conditions are more controllable than the aquarium itself.
In 2013, I began a new 33 gallon marine aquarium with a 20 gallon sump located under the aquarium. For controlling its evaporation, decided to try the e-Fins Easy ATO Kit, which I'm now very happy I did! Basically, its consists of six items; a small, but very powerful DC powered water pump; a small 200 micron filter bag to house the pump to keep it always free of floating obstructions; pump power supply; six feet of flexible clear hose to connect the pump to wherever its flow is needed when turned on by the float; a small three connection circuit board; and of course, the float and its magnetic holder.
Setting up the device took all of five minutes, and it has been operating flawlessly for several months. In my setup, the water reservoir sets next to the sump, yet it is said the pump can deliver water to six-foot heights if required making it ideal for nano aquariums on tall stands that don't have space in their stands for sumps, or those aquarists who do not prefer to utilize sumps.
As with all float ATO's, its prudent to inspect the float mechanism at least every few weeks. And during this inspection lightly wipe the float and its stem with a soft bristle brush, which takes all of about 5 seconds.
There are many float type ATO devices currently available, but can honestly say I've found the e-Fins device very satisfactory, and quite reasonably priced.