Red stems and dark leaves

More magnesium is usually the solution.

Hey Growmie, Purple stems and branches could be caused by several things. Some of the natural causes could be plant maturity and genetics, there could also be environmental factors like lighting and temp fluctuations at night that can also cause this. Something else you can check would be if this is from a deficiency, such as Phosphorus, Magnesium, Potassium, or Calcium, all these deficiencies start with purple or red stems and then it starts getting worse.

Red stems Could be a number of things and a could be nothing. Darker Leaves are typically from excess N. Some genetics cause red stems when exposed to strong light, Could mean its lacking magnesium or phosphorous and boosting P past its needed amount can lock out Mg as well as other macros. But you are likely to see Mg defenceless in the leaves as well with yellowing in between the veins. but it takes some time to show and once you do see it it normally means its been deficient for a bit. Temp swings can also cause some reddening but you would expect to see this on the leaves as well. PH is also a factor you should check, if your PH is fine then its not this, but PH can play a role in why it can not take in P or Mg I don't think this is much of anything to worry about, the excesses N can be reduced a bit, Always water with some runoff to prevent buildup, and don't over do it with AN, they are way to expensive and better products out there for much less. "Jacks, Tera aquata,Metrop to name a few salt based feeds" Maybe invest in a cheep EC pen so you know the concentration of what you are feeding and can monitor the runoff to confirm its not to much. Good Luck!

I wouldn't worry about the purple leaf stems. Every variety of weed i grow has that trait. Some say it's a sun tan from light exposure. Purple stalks are another story. If your leaves are very dark green they are loaded with nutrients and you might want to dial your mixture back by 25% I reduce my feed chart numbers now because i burnt the shit out of my plants because i failed to take into account the high ppm in my tap water. It never hurts to reset your soil now and then too by flushing out built up nutrient salts that can potentially cause problem down the road in flower where you don't want any issues!