Lettuce leaf tip browning

[Summercreek]

I have recently started growing hydroponic head lettuce and have a problem with browning leaf tips which starts at about 2 weeks growth and continues to get worse. Attached is a picture showing the browning starting and also some vein browning. Does anyone have any ideas about correcting this problem?

Summercreek

[willard3]

Reduce nutrient concentration to start.