My grow mentor, Chickengutz, said if you MUST change out the rez, leave approximately 1 gal of the old nute solution in with your addition of fresh water. Eases the transition to clean rez water. Wait 1 day and than go 1/2 dose and work back up to a full dosage as you go back to your old feed schedule. If you are ready to start your flush, leave the 1 gal of old solution in and just add straight H2O. The samll amount of old nutrients will dilute to almost non existance. And than your can add your Clean out solution with no issues. I'm quoting a recommendation...and I have never tried it..but will when I start my flush.. Hope this helps. Good luck friend!