I think pH 6,8 is a bit too high. Should try lowering to 6,5. Flush the soil with properly pHd water. Salt buildup from those fertilizers could lock out phosphorus and nitrogen. Phosphorus deficiency could couse the purpling too. I'm no expert. Just saying what I think. Pics would help
6.8 is
IDEAL in soil. But even so, a variance of no more than .3 points isn't gonna matter one
iota in soil.
Without pics, it's
impossible to give any kind of real advise, much less make an intelligent diagnosis of any problems that
MIGHT be going on. Purple stems and all, if you're flowering, I'd have to say off hand that you are most probably seeing normal leaf shedding.
Dont over engineer this shit. pH of the
SOIL itself is the crucial value, not the pH of anything you put into it. If your soil is properly buffered (limed), the pH WILL NOT CHANGE significantly enough during the course of the grow, and certainly not enough to cause you any problems. The soil is gonna change the pH of anything you pour into it to the value set by the soil and lime. So if it's right at the start, you'll be right at the end. FAR too many people out there have not a clue what pH is or how it relates to nutrient take up, yet thats the first thing everyone screams.
I doubt very seriously you have any real issues going on, but please post pics, just to be certain.