Auto topoff via Air Pressure.
Here is what I use, I have a sealed 5 gallon water jug with two openings. One large for filling and pouring, and one small for venting. So I took the cap to the small hole and made two holes in it, one for Air IN and the other for Water Out. I run standard airline tubing from a small air pump to one of the holes in the cap. Then I run another line from the bottem of the tank through the other hole in the cap to an airline distro station on the tank. Once it is all sealed and the air pump is running water will begin to flow up the tube that leads to the main tank distro station. Now adjust the flow to match your evap rate and you are good to go.
You can also run this same set up with a float valve instead of the air distro station, but thos are expensive. (24 bucks vs 2).