My experience has been that my salinity raises, not lowers. Every few WC's I compensate w/ a lower salinity. I know certain brands of chemicals like magnesium and calcium have added salts to make up for losses. Either way, it's nothing drastic and regular testing will keep you spot on. Most skimmers will also allow you to skim "wetter" or "dryer". Some people even do their water changes by skimming very wet.