I am talking about the principle of adding ions to the tank
you cannot measure home so you do not know if they are used
up and need replenishment

So am I. What did you think I was talking about?

You were talking about: importing of trace elements that get
used up over time. Check the posting history and you will find
this quote:

Don: Import of (possibly unknown) trace elements that
get used up over time.

Your response: I use Kent Marine Essential Elements.

That is the reason for my question: how do you know that
given ions are used up and in what extend ? How can you
know that .4 ppb of Iron brings this ion back to normal
concentration if you do not measure iron consumption in
your tank ? Same applies for other ions in nsw...

Your correct. I don't know. I have to make some assumptions. I still
think it would be difficult for me to overdose given the amounts I am adding
and the frequency of addition.

You add elements you are "unable to test their concentration"
of every time you use your salt mix. Don't you?

Here is the important difference: every water change brings
water closer to the original ions concentration mixed at factory.

Yes, but you said: "I would not risk adding or removing elements I am unable
to test their concentration."

You are still adding elements that you are unable to test their
concentration for. Are you not?

The advantage you have (only one I can think of) with performing water
changes is that you are removing and replacing water that contains essential
elements (of unknown amounts).

You are making an assumption also though. You are assuming that you are not
accumulating any of the elements you are adding (in your saltmix or
supplementation) in your aquarium (those not removed with a water change),
which would result in an overdose.