Aqueminianswered grow question 4 years ago Sounds like salt build up, flush your medium and put a fresh batch of nutrients. Excess nutrients can block other nutrients from being absorbed. Your pH should climb or stay the same. If your pH drops and keeps dropping then there's probably to much nutrients for the plant.
Also that could be light stress, or heat stress as it's mainly the top of the plant.
On another note, how far along in flowering are you? If your near finish then yellowing or loss of color is normal.