You know the red stems, I had that most of a times and after 4 years of growing I am pretty sure that, this has something to do with one of the two : CAL or MG.
I am running new grow (2nd week)and as a experiment I mixed dolomite lime into the soil to get extra calciume into the soil, and added 1/4 of bannana on the bottom of the pot for extra P at later stage of the grow (see if it will work) to shake the reddening of stems because to me it looks like it is sign of deficiency