As meklo stated ,.. Yes you turn it off while doing you pwc
,..then refill the tank ( sump) to that exact level before your water change ,..(most of us prob just have a line drawn on their sump indicating this ),...if you do not refill to this level( under fill ) then the ATO thinks that your tanks water has evaporated and will continue pumping in new water until it reaches the optical sensor which will shut it off,..... The problem here is that in a saltwater tank your salinity will change and will become diluted ,decreasing your salinity....if you overfill your tank and cover up your optical sensor while doing your pwc
then your water will have to evaporate until it gets down to the optical sensor before it'll start to work properly,..that will cause your tanks water to increase in salinity.
I've been using them for a few years now and they work very well . , don't forget to adjust your other sensor for the high water mark also,.,very important too ( I found out the hard way ) .
I have found that the best part of the Tunze ATO is that they're electronic and the worst part of them is that they're electronic ,.. Lol,..( and pricey.)