Hawaiian Snow issues

I have no exp. with this variety until now. Greenhouse Seed stock. DWC. PH 5.6, PPM 800 Lucas Formula, temp. 75 F. 400 hps 20" from tops.
Photos could be better, but the symtoms are uniform in each plant. Leaf sides curled up, leaf tips yellowed and curled up, necrosis in later stage.
It looks like heat stress but at 20" from the lamp I cant see how. Used to grow c-99 and it had no such issues.

Wow, dont get me wrong, but if this was the original Overgrow site some sort of guidance would have been give at this point?! How disappointing.