How about an overall picture so we can see the rest of the plant to judge the healty. There are many times in the life cycle of the plant when one leaf turns yellow only because the plant has used it up and no longer needs it. Ph of 6 in a hydroponic system is fine, 5.2 - 5.5 is more like the target but 6 is not out of the question. When was the last time you did a nutrient change?