Honestly that looks like some type of nitrogen problem. Yellowing leaves is usually caused by either nitrogen overdose or nitrogen lockout. You say you have your PH at 6.8? I'm guessing by that your having nutrient lockout from your PH being too high. I would try balancing that down to 6.0. 6.8 is a bit high for that type of grow. What type of nutrients are you using? if your over nuteing the plants that could be an issue too. Do you have a PPM meter? if so what is your PPM at? You want your PPM to be around 1200 in the first weeks of flower. I would say its a safe guess that its caused by your PH being too high though. Try bringing your PH down and let me know if that helps.