Of course I'm more of a "questions asker" than a "questions answer" person but you SHOULD not have to add much, if any salt.

The salt stays in the water until it gets splashed out or pumped out with the backwash water. It doesn't evaporate. I added a couple of bags to mine for a maintenance dose....but you should test your water to see what the current level is before adding.

I'll let Ben and Co. answer the chemistry part.