salt placement

[Arok3000]

Taking a fairly extreme case.
Let's take a 90 gallon display with a 40 gallon sump, sump isn't full to the brim and there's rock in the display, so call it 100 gallons actual water.
Say you have an extremely large skimmer cup (14 cups in almost a full gallon!) and it fills once a week with not much gunk and pretty much all water at 1.026.
After one week, your sg would drop from 1.026 to 1.02574, two weeks would be 1.0255, and three weeks 1.0252.

That's pulling a gallon of skimmate a week of what appears to be a relatively unrealistic sg of the skimmate.
I would venture to say that although your skimmate does no doubt contain a small portion of saltwater in it, the design of a skimmer should drain the saltwater and only skim the organic crap for the most part, and anybody running into a significant swing with their sg because of their skimmer, no matter how "wet" you skim, has a serious skimmer malfunction, or the cause is probably somewhere else.

[craigwmiller]

I will totally concur with skimmate being a drain on salinity... The only way it would not contain salt would be if it was distilled And since I doubt your skimmer(s) are pushing foam near the boiling point, the liquid in the cup has not been evaporated/condensed during the process and thus is most probably not pure water. So salinity is thus decreasing because top-off water is pure and diluting the tank.

Same applies if you are dripping new tankmates -- I always add to the tank about how much I plan to drip out of the tank prior to starting acclimation so that my ATO doesn't run extra and dilute my salinity (which as everyone I have set to a specific level on purpose)

[cuz]

just my opinion but the skimmer is and will continue to be the last thing I look at for SG fluctuations.

Maybe if you were running a 500g skimmer on a 5g nano?????