Salifert are streets ahead of API, I have no clue why everybody recommends them.
Salinity will go up and down with evaporation, but not to the extent you suffered, not unless you have been literally boiling it?
Are you skimming especially wet? That will drop it a little as it will remove more salt.
Nitrates at 50? Do you run any chemical filtration? I'd suggest tossing in a sack of purigen, it may help a little, plus water changes will assist.