from what i know purple stems can be genetic or due to a lack of nutrients.
it seems to me that nutes you are using are pretty strong, espically if you soil is good and rich. if you over feed your plants the nutrient salts will build up in the soil and be deemed unusable by the plant.
my advice would be to leech the soil with straight water, wait a few days
and mix up a 1/2 strength foliar feed. use this till you see a change.
wman44 Reviewed by wman44 on . Purple stems? i kno a lot of people have asked about this but here we go. conditions: -80 degrees F. at all times(sometimes 75) -Small grow room.(6 feet tall by 2 feet deep by 21 inches wide) -using a T5 compact fluorescent bulb with hood(gunna switch to a HPS lamp when sex shows) -plant is about 3-4 inches from light -only one plant is affected by the purple stem -looks pretty healty -using 24-8-16 with micronutrients(added 1 t. to a gallon of tap) Rating: 5