3 Week old WW - Heat stress or MG Def?

Is this more likely Heat Stress or Mg Def or ?

All 4 of the widows I have are showing the folded curled up leaves. The pictures show just one plant.

Nirvana White Widow - 18 days in veg.
FFOF 1.5gal container.
76-78F / 45% RH .
No nutes - gave one feeding of 5ml cal-mag
PH water is around 6.5-6.7

They are thirsty. When the edges curl like that the plant is trying to conserve water because it is lacking. Give them a drink should take care of it.

I've got a very similar problem right now too and I was contemplating the same thing..I'd like to hear more opinions..

With the twisted leaves it looks as though they could use somemore cal mag.

I think i have the exact same problem. But based on the answers given i did not find any solution. So is it Heat Stress, or lack of MG, or lack of water ?! can someone please elaborate?!