Northern Lights x Big Bud Auto

Less leaves = less energy produced = slower/smaller growth.

While Nitrogen (N) toxicity may produce darker foliage and brittle stems the top foliage remains dark green as well due to increased chlorophyll production it is not your plant's case! I actually believe that this is a Phosphorous (P) Toxicity - New growth develop thin and distorted blades, small internodal spacing, leaf tip chlorosis, and several micro-nutrient deficiencies (Zinc, Iron, Calcium, and Magnesium) that causes the discoloration on the top of the plant. Try reducing the total fertilizer dosage to about 3-3.5ml/l of the "Grow" and "Micro", 2.5-3ml/l of the "Bloom" fertilizer (increase to 3-3.5ml/l when healed), and as for the "Big Bud" you can leave at current dosage or to be sure you can even reduce it to about 1.75ml/l Hope this helps! Good luck and happy growing :)