I'm not sure of your question. maybe it was answered for you already?
If you mean, you don't wnat the steph of your mix in the raz to rise then. It shouldn't.
Unless it is to high to start with, in which case the plants are drinking more water than they are taking in Nutes which very bad.
I aim to have it the other way, and set my Nute streth so that it slowly goes down.
As for topping up, I top up with plainn water and change the rez once a week.
Other poeple also top up the Nutes and don't change there rezes so offten.