Leaf Problem Help plz

Hi everyone

Hopefully someone can help me with my first grow and it's problem.

Thanks!

HYDRO GROW

Strain - Sativa Super Silver Haze
# of Plants - 4
Grow Type - Hydro
Grow Stage - Flower
Setup - DWC
Light - 600 Watt HPS
Nutrients - Mega Mass
Medium - Hydroton
PPM - 777
PH - 5.5-5.7
RH - 45% to 55%
Room Temperature -68 via A/C
Tent Temp - 77 top of canopy
Solution Temperature -67 using chiller
Room Square Footage - 16
Pests - None Known

Problem: I'm having a yellowing and browning of my leaves on 3 out of 4 plants.
Two plants have stunted growth. One is being a total champ and seems unaffected. Any ideas?

Russet mites. Put the leaves under a microscope. A magnifiying glass will be worthless.

Thanks for the suggestion.

Just put it under a microscope and went through 6 leaves. No bugs at all. Someone suggested rust?