I am growing with aeroponics and I noticed lately that some of my plants seem to be getting a light green/yellow in the middle and that the new growth is coming in all light green. There are no spots on the leaves and other than being light green/yellow seem to be doing ok. These plants are in veg mode with a 20/4 light cycle and two 600 MH lights over them. The room temp is a bit high at 88 degrees. The plants are approx 14 inches tall. I am using "Dutch Master" nutes and change it once a week. I have never flushed the system. I keep the The PH level at 5.8. I have attached a couple of pics.