Interested in this as well. I have a 35 gal waterbox, and am about to buy an ATO system and reservoir. Debated going with a 15 gal reservoir over the 5 (only choices) but I thought 15 gallons might be a little excessive for a 35 gal tank.
Depends on a bunch of things. What size tank (dimensions)? How much surface agitation will it have? Temp in the room? Does the tank have a lid? Will the ato have a lid?
My 4g tank with a full glass lid has almost no evaporation (if I measured evap it would be in ml)