Funky lettuce check out picts

[bigwhale]

Have a very small system . 5 gal bucket system flooded. Bubbler, 150 watt cfl 6500k about 9- 12 inches above plants can feel some warmth on foam , rock wool media with cutouts in 1inch foam, and root baskets. used mircle grow tomatoe version mixed half of what the directions say,if mixed full caused 1st set of plants(which were pulled ) to have leaf burn (yellow tips). Lettuce and radish plants. 2 of each. Radish loves it and grows leafy but not real dark green leaves,and many clean white roots. Lettuce grows slowly and very dark green leaves, with smaller roots, roots are white and healthy, Total PPM is about 600 with our tap having about 75 PPM. PH is about 6.9 They are both in the same bucket. Same light source, 12 hrs per day on timer. about 67 F air temp.Noticed new leaves with the crotch of plants are curling downward and black brown tips. leaves appear and feel hard and chrinckly on tip areas. Mostly on lettuce. Water is mostly clear and has very slight earthy oder .No visible alge present. Whats going on here? Thanks