xlr8 Thank you for your insight it is very much appreciated in fact I am going to try your suggestion on DM "zone" out on next water change. fyi.. nutes I run are as follows.... GH micro grow bloom and during flower I add liquid kool blooms . I fill my res to 55 gallons and mix for 45. so that leaves quite a big buffer. My ph is stable I use GH up or down as needed but mostly down.