Leaf tips turning brown

[desert girl]

I am on grow day 26. I currently have 3 plants growing. They all germinated very well and are in a closet grow. All have the exact same grow medium. Ffof mixed with light warrior, perlite, and lime. Using 6500k cfls. One for each plant kept about 2" from top center of plant. Extra lights in between for side light for new foliage growth. 1st 3 weeks only plain water used. Misted daily as pots were flushed prior to planting germinated seeds. Fed at 3 weeks using 1/4 strength grow big. Great drainage. Again misting as top soil gets dry. Good ventilation humidity difficult to maintain but avg RH 40%. One plant, an auto Anubis today started showing brown leaf edges on. 2 leafs. Other. 2 plants very healthy. Any ideas or suggestions? Thanks in advance for your help.