HMMMMM. Well I am in the transition week to flowering, although 3 are older, just over a month, and 2 are only a week or two, and another is somewhere in between. 730 ppm levels, I gave the 10-12 gallon res an extra tablespoon of micro due to what looked like some manganese deficiency in some leaves. I am highly interested that you said your levels change from 5.6 to 6.5 IN 24 HOURS!!! Holy crap is that normal? How many plants, how big a res? I have 6 in a 12 gallon res. It sounds like that could totally be it. I am Adding around 10 tsp of micro, 10 of bloom, 10 of grow, 30 total (as recommended by advanced's website for light feeding a 10 gallon reservoir), to my 10 gallon reservoir for six plants. Does that sound low to you? I think it may be. Could low nute levels cause it to change ph that much in just a few hours? Keep in mind my ppm levels don't fluctuate all that much, no signs of deficiency other than the above.