The Solenoid will be on timer. I will time how long it takes to fill up the 10g and use the float valve as a backup. The float valve I have on the 10g I got from BRS where it stops the water flow completely when closed (like a toilet float valve). I made a mistake on the drawing, this float is not hooked on the apex since its self closing based on the water level but might put a safety float on there also just incase.
As the water drains, the ATO will kick in naturally. I have 2 sections in my first chamber (1st section slightly overflowing to the second section). 1st section will have the drain (where the drain of the tank goes directly to) the 2nd section is where the water ATO fills. This avoids new water accidentally getting sucked out. I will have pictures so its more visual and maybe a video once its up and running.
As far as water levels, evaporation, etc. I will have to experiment with it once I have a week or two of run time and I can adjust my programming and also adjust the % water change and might have to fill the 10g 2-3x a week. But my baseline for now is once a week.
All the automatic water changes I see if where they all have "well" water and do not need to have a reservoir to cure and age the water. This is the idea I came up with and hope it works out.