Deep green leaves with downward curling tips point to too much nitrogen rather than a shortage, which is the opposite of what most people assume when they reach for more feed. Growth slows and the plant turns brittle, and the deficiency chart on https://kushindicaseeds.com puts toxicity symptoms next to shortage symptoms so they are easier to tell apart. Flushing with plain water usually settles it. Cutting feed strength by a third afterwards prevents a repeat.