Leave Sides Curling Up. (Not Heat Stress)

My 2 Starkiller plants both have this.

Plants-Starkiller (Rare Dankness)
Light- 600w MH
1/ Bubbleponics (Top-Feed DWC)
1/ Coco Coir (No Perlite)
Nutes- GH Flora Series, Superthrive, B1, Cal-Mag, & Hydroguard.
Temp 77-81F
PPM- 450
PH- 5.8

(I just topped this plant but the problem came before this. Plants continue to grow but I'm kinda OCD)

What is your humidity at? I had the same prob because my humidity was 15%.

Your title says not heat stress but it is classic heat/moisture stress

temp & humidity combine to make what is called the Heat Index. Since your temps appear to be inline your humidity must be off

Normally they curl up like that because the leaf is searching for more moisture.