Hey cola so I feel like no one ever mentions that phosphorus deficiency can cause the reddish stems as well and along with the lower leaves yellowing and discoloring so i just wanted to add that. I see your ph is much lower than last week which is great because if it is nutrient related you probably fixed any possible lock out. I agree also with the calcium/magnesium..well more so magnesium since calcium has a tendency to attack new growth or actively growing leaves, like vegetative growth. I did look at a variety of other sites and sources growing northern lights and i did see that many of them also had reddish stems, so it may be a strain specific trait also. Keep up the good change and watch for progress, if the red stays when you keep your good ph and nutrient line up then it's probably just genetics. Happy growing โ๏ธ