Whan you change the res you add your fresh nutes then and ph. then you monitor and adjust as the week goes on but will mostly add fresh ro water. I only change my res every 2 weeks but a good gauge is to change the res once you have added thefull volume of water back. So if you have an 18 gallon res and add 2 gallons a day then you would change the res every 9 days. So keep track of how much water you add back. No need to throw out good nutes. Another way to tell if your res needs changing is if the ph goes wacko and it becomes hard tocontrol. Hope that helps. Maybe more info than you were looking for.
