I'm a noob, too, but I disagree -- that looks like classic nitrogen burn. That also looks like MiracleGro potting soil, no? If so, I'd be even more sure that it's N burn.
N burn starts on the lowest leaves.. sometimes it will stop there and the plant will move on from it, other times not. If it were mine (and if I'm right about the MiracleGro) and it started showing up higher on the plant, I would transplant to better soil -- Foxfarm ocean forest, if you can find it (the Foxfarm website has a searchable map to find dealers near you).
The light does look weak, although I realize it's hard to take good photos sometimes under a strong HPS or whatnot.