ATO diluting salinity

[Arok3000]

From what I've gathered, you're overflow isn't tuned precisely, and is causing a slow change in the sump level. Albeit a slow change, it is a change nonetheless.
There isn't an ATO on the market that will compensate for a mistuned overflow.

Is your overflow internal? My herbie is internal, and if it were to swing way out of tune and drain the over flow, I'd have an extra 4.5 gallons of water in my sump. That would cause an extreme difference in my salinity also.

I'd make sure your overflow is 100% consistent before you go ahead and drop some money on an ATO that isn't going to help your problem at all.