Couple of question(s) about a curling leaf, nutes and pots

So I have a three-week old with about 6 serrated leaves.. One of the leaves began curling a couple of days ago. Strangely, it was not from both sides, but just one. I'll post a pic tomorrow if neccessary. I have read that it could be over- -heat, -water or -air. So, I removed the (very light) direct fan, moved the 4-tube, T5s up from 12-18" and am leaving it alone. It, they, are on a 20:4 schedule from the sungod. Though, I haven't yet established a consistent watering schedule with this guy because I've been getting so many conflicting opinions on the best schedule; though I think I understand now. Anyway, I guess my questions are: what else should I be doing to protect this leaf? When is the best time to begin using nutrients for these guys? (soldierbud is 6" with his curling leaf, the others were planted today to replace a few weak links). Lastly, at what size is it a good time to move them into their final (flowering-size) pots and what size should that be (they now live in 6"D clay pots). Thanks in advance for any help you can offer.