The ATO goes in the sump. The display tank level will remain constant since the tank level is determined by the overflow box level but the results of evaporation will be seen in the return pump section of the sump. In my case my return section can drop 6 gallons before I have to add top off water but it still required adding water no less than every other day. The sump adds more volume but you still have evaporation, often more since you now have more surface area for water to evaporate.
