I keep a similar size tank.
In small tanks running an auto top off there are a lot of factors that can constribute to slight salinity fluctuations.
Some examples from my tank are: a drain pipe valve being opened or closed slightly, filter socks filling up, overflow/sump baffle teeth becoming slightly obstructed, and the extra sodium chloride formed as a byproduct of two part dosing .
for me, the ones listed above are bigger contributors to salinity fluctuation than the skimmer pulling salt water.
Maintenance related to the aforementioned factors and semi-regular water changes seem to keep things mostly in check.
sometimes if my salinity is running a little low, I’ll add some salt water then let it evaporate until the sump is back to its normal water level.
I’ll repeat this process if necessary.
So basically, I’d just keep an eye on salinity as long as you aren’t emptying your skimmer cup multiple times a day or something.
