CHRONIC OVER WATERING!!!!!! See the lighter yellowish green nearest the stems, and that the stems are purple is a strong indicator of slightly overwatering over a period of time. Leaves look full and hamper leave respiration, which also causes the edges of the leaves to slowly die. Reduce your feeding and flushes so that you are only giving them water every 4-5 days apart, and keep them in the right sized pots for your plant. Though you do not show us the plant in the pot, I suspect like most hobby growers, they place their plants into huge pots thinking that they can save time by not transplanting so often, but then they water the entire pot with soil to the point of run-off, meaning that the whole pot is now saturated and the roots have not even grown out to this part of the pot yet. This causes nutrients (salts) to build up in your medium over time and by the time the roots get to that part of your pot the soil is hot with nutrients and causes nutrient burn, which is #1 way of killing a cannabis plant.