What medium are you growing them in? What's your root zone looking like? The fan leaves that are dying - are they fading to yellow first, or just going straight to dead? How often are you watering?
Edit: Just took another look at the picture. The leaves that I can see don't look healthy. How long has this been going on? Could be several things, hard to tell for sure. What can you tell us about your nutes and everything else you're doing?