AQ salt, used for specific treatment- you would be doing daily 100% water changes, adding in (water conditioner and) salt each time making sure to dissolve it first to avoid burning the little ones. You would use it for no longer then 10 days recommended, 14 days at most for certain cases.
You wouldn't want to use it on a normal routine as in time it can cause internal problems such as kidney failure and resistances to it by both the fish and the bacteria. Also not recommended to use with live plants as it can kill them easily.
AQ salt is good to have around, for external problems such as Ich, fin rot, etc. I know you just asked about water changes, but figured to cover the whole of it :)