If you are having fertilizer salt build-ups then it won't take long before that flush solution's ph drops and the ppms go up. You can flush with ph adjusted water its just that the distilled or RO water gives you a starting point to see exactly how much salts got flushed from the medium. I flush mine for several hours before every other resevoir change, which is every 14 days. Haven't had any problems with doing it this way yet.