I'm going to start very slow, very basic, and work toward the lesser known things.
1. your salt mix is not just salt. It's dry seawater. And that means not only salt, it's calcium, magnesium, iodine, selenium, boron---read the label on your salt mix.
2. evaporation never evaporates the minerals. It only evaporates the water.
3. animals and plants suck up part of the minerals. So TWO actions unbalance your tank---evaporation increases the mineral concentration---and...