Baby plant probs - noob

hello,

I have 3 plants currently with problems - I sprouted them in the dirt pellets in the mini green house - 2 shot up quickly and one is slow to grow - 2 weeks in and i transplanted to rockwool with rooting gel - one got a little shocked the NL C99, but bounced right back after a couple hours -
I have been feeding a half strength BC hydro solution (Boost and Grow - no Thrive alive yet) for the past week 3 times a day for 15 min each time - i have an exhaust fan big enough for the room, fans to move air, an air conditioner and they have been given 18 hours under flouresents - I have been checking PH a lot and regulating when needed but it hasn't been that off,
the leaves started yellowing 2 days ago and one is starting to get brownish color on it

Please help