Also been running the GH for awhile, in my dwc I adjust to 5.6 when changing rez and don't adjust at all after that, it's at 6.4 usually when I change my nutes out. Most of that raise is prob from top offs, seeing as my tap is right around 7.2! The drift in pH I believe helps the plant to take up the full spectrum of nutrients available to it.

@5.6 your getting the Calcium and Manganese.

@5.9 and above your Phosphorus and Magnesium

Everything else is available in between, you need all of it.