This reply is in regards to your concerns over your ato, an ato should have float switches with it which would only turn the pump on and add water when needed so I'm unsure of the cause for your concern. I suspect it is evaporation that is causing air in your overflow so an ato is probably gonna be a nice addition. I'm not sure of your experience and please don't take offense here, the ato doesn't throw your salinity out of whack, it does reduce it some which will put it back to the proper level. Evaporation only reduces the water volume, the stuff dissolved in the water stays and will become more concentrated (salinity increased) so with topping off you reduce the tds
back to normal range.