Changed out the res today - here's a pic of the roots in a near-empty res. To me, looks like some slight staining but a little slime here and there too...
This res change I went to 0.95EC; pH 6.2 w/ RO water, cal/mag, nutes, 2.5ml/gal hydroguard, Great White @ about 0.75g/gal. - in that order.
I don't think this relates to the pH crash issue - or maybe it does - but about a week ago the fan leaves on one strain (2 of the 4 plants) started turning purple - but only on the leaves closer to the lights (still 25 in. away from my LED (750w equivalent HID)). I first thought it was K deficiency upon doing some research, but where the leaves of two plants touched each other, I could lift up the top leave and the underlying leaf was perfectly green where no light hit it, but almost looks a burnt purple color everywhere else.
I have since raised my lights and done some LST to move the taller colas away from the center and down a little - looks like they continue to get purple, so maybe it's just the strain, but it does look like they're stressed in some way. I guess we'll see...buds still developing nicely - the other strain is having no such issues