I know reef central has several threads on the subject. Curently im adding manually till I repair my top off pump.
I made my own ATO, works great. I never even look at the level in the sump and skimmer as I know it's going to be the same. The only time it really has to open up wide is if I put less water back in after a water change than I took out, but that is never more than 1/2 gallons I would guess as I use five gallon buckets
