This may be a stupid question, but even after two years doing saltwater tanks I never really thought about it. Does skimmate have salt in it? Or is it just water that your ATO will replace?
Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
It is skimming the water from your tank so yes it does. Skimmers do not work on fresh water tanks because only saltwater foams.
I asked this same question myself only yesterday.
The answer is it pulls salt+water from your tank, and the ATO will refill it. However compared to the overall volume of the typical tank and daily evaporation, it's a very small amount that can be compensated for quite easily with regular salinity checks and top ups, frequency depending on how wet you are skimming.