How big is your tank, Narkon? I don't think salt creep can cause your tk salinity dropping that fast - unless you have a leak and a auto top-off w/ fresh water.
Check your skimmer too- if you have one. Did it overskim w/ very dilute skimmate? if it does, and you refill your tk w/ fresh water then you essentially dilute your tank salt water there. So adjust your skimmer, and may want to replace the dumped skimmate w/ equal volume of newly mixed saltwater.
I wouldn't recommend to (auto ) top off your tank w/ mixed saltwater!!! .. Your tank salinity could go to the roof in a short time if you have a good rate of evaporation. The salt itself does not evaporate - only your water. So, your tank salinity supposed to go up as your water evaporates- and hence you need to add FRESH (DI/RO) water to keep the salinity level stable. So my advice is to find the problem first (overskimming, leaking, bad hydrometer reading, not so well-mixed saltwater...), rather than adding saltwater to mend the problem.