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 :)