you dont have to change the res but it is advised. the nutrients are poured into the water and (in theory) dissolve, the plant then sucks them up with the water through its roots along with oxygen. problems will occur if ur res temps fluctuate too much, if ur water ph is off too much or theres not enuff oxygen, when this happens the plant responds by not taking all the nutrienrts that it need, so your water has a high concentration of one or two of the nutes, you could just continue to top ur res up and feed as normal or u could empty the thing, give it a little wash down and start a fresh feed, the benefits in the latter should be obvious. i live in Scotland and our tap water is exceptional, some of the best on the planet, as aresult we dont really have to PH it and as its it has reduced calcium, magnesium and other shit this means that nute dissolve very easily, hard water is the oppposite, with it you get limescvale and all shorts of shit that fucks up and clogs your plumbing. i dont even bother with an ec/ppm meter. the choice is yours buddy, hope it works out.
