Hi all... Can someone with more experience take a quick looky and let me know what you think. The lower, large leaf is curling but the other leaves are not. Out of 3 plants, only one is doing this so I'm not sure if it's heat or because it's just a "lower, large" leaf. The plant is 18in from the 400 hps bulb and does not feel not when I put in hand in under it.

The plant in the picture looks yellow but it's only the photo. It's actually a dark healthy green.

Thank you in advance.

Veg: 11 days
System: DWC
Temp: 75-80 F
Humidity: 35%
Dyna-Gro Nutes @ 460ppm
pH: 5.8 - 6.0