Mixing aside, there's very little chance that any salt water (even NSW) can maintain NSW levels of certain trace elements unless you do massive water changes (like 50% daily). That is because some trace elements are consumed rapidly.
What folks strangely disregard is that foods are likely a far bigger source of many of these elements than are water changes.
That said, foods often do not add enough to maintain NSW ion levels of all trace elements.
The underlying and unknown question is how low can these elements (such as iron and manganese) get before become a problem for an organism you want to keep.
IMO, a trace element additive is a fine thing to experiment with. I'd be sure to get one with both iron and manganese. I think Tropic Marin A and K may be a good one to experiment with.