You are taking out a more concentrated level of minerals than what is being added. If you are using RO to top off then the amount is = or < than what you put back in a WCsorry, i have hard water. every time you add water you add minerals. if water evaporates in a finite aquarium every time you add water you add minerals they compound.
Calcium has an important role in the biological processes of fish. It is necessary for bone formation, blood clotting and other metabolic reactions. Fish can absorb calcium for these needs directly from the water or food.
Personally I think you're overthinking this.ok, will top with distilled water. thanks,