I've got a lettuce raft that's been happily growing for the last month an a half. My romaine plants look good but having some browning on the tips of the butter crunch. It started with one plant a week or 2 ago and now a few more are doing the same. It's on the tips of mostly the inside leaves. The nutrients are at about 360 ppm and PH 5.5 - 6 somewhere, under regular florescents. I keep the lights about 1-4 inches above the plants. Any suggestions?