It doses the plants before it's removed, there's some debate whether it removes traces that much or how much. The main part is using it in the start up phase, not all the time. ADA does not add ferts for the first few weeks typically either.
I'll use carbon as well, namely for color removal and Zeolite for NH4 removal.
After a month or so after each is spent, then the materials become bio media and are just left there.