6: 400w hps 18/12
Well, a 30-hour day is weird. 18/6 maybe?

14) How often are you watering between feedings, and how much per watering?
And you didn't answer this question. I think the problem is lurking around here somewhere. That pot is pretty big, too. I'm thinking soggy soil > whacky soil chemistry > nute lockouts, etc.

Keep them younguns pret-near rootbound as they grow, so the roots can drain the soil better. A coupla inches of pebbles, broken pots, etc. in the bottom of that pot would have helped it drain better till you got some roots goin'. (There are drain holes in the pot, yes?) Was it rootbound when you transplanted up to that pot?

Anyway, without getting too technical (because I am fundamentally unequipped) I would guess it's "Overloved" via too-frequent watering, or some variation of that theme.

And Bonedaddy's right. 400HPS is a lotta light going to waste, and a lotta heat for a youngun. Prolly have better luck with some 5500-6500K CFL's or some Cool-white fluoro tubes. Some folks have good luck vegging under HPS. I'm not one of 'em.