as time goes on, the nutes in a res go........

so once you have freshly put nutrients in the res, i know that they say you should put in new water every week or two, but during that time water and nutrients are absorbed. AFTER ABOUT TWO WEEKS LETS SAY, WOULD THE NUTRIENTS BE CONCENTRATED OR DILUTED IN THE RESERVOIR? any responses are greatly appreciated and are always worth time!

I keep 30 gallons in my rez and I drain it every two weeks. I also top it off with fresh water every other day(or so) and check for PH/PPM and adjust as needed.