yellowing leaves and red stems

This is my second grow. Using soil, properly pH'd water with botanicare nutes at label levels, week 5, northern lights, led grow lights. I'm getting yellowing leaves on one of my plants - the other two have one or two yellow leaves at base - I expect those. These are more toward the top of the plants.
This is only happening on one of three of my northern lights. Ideas of what might be causing this? Is it critical?

Can you turn off the HPS and take a better pic? It will help with the diagnosis.

Hmmmm this is the first picture I saw that looks very similar to my actual issue
I am in week 2 of flower!!
You know what it was in the end?
You was able to sole this issue?

Ive had red stems on each of my first two grows. Never had a plausible explanation or cure, but havent noticed any harm they've caused either. As for yellowing leaves, I was having a nute lockout issue. Started watering til 15% runoff and that seemed to solve the issue.