First time grower needs some help.

My plants are about 2-3 weeks old and recently I've noticed some brown/copper spots on the lower leaves. They started near the middle of the leaf in between the veins as a single brown/copper spot and started spreading though the rest of the leaf. Now on one of the most effected leaves some of the brown/copper spots are turning yellow/white (but that color isn't spreading through the leaf it is just the spots).

I have two plants (one of them didn't have any of these sympotms until today - now there is a single copper spot on one of the lower leaves of this plant as well) in a closet with a 400w MH lamp. It's currently about 21 inches away from the top of the plant. I'm on a drip hydro system using GH 3 part fert system. I've followed the recommended amounts on the back of the bottles but at 30% strength. I'm growing in rock-wool and the system is on an automatic watering for 10min every 3 hours.

They've been growing great. Good color on the leaves; every day they seem to get bigger. The stems are growing in nicely; starting to thicken up. New leaves are coming in quickly and heathly. Everything has been going well until these brown/copper spots.

I tried to take a picture but it would come through clearly. At first I thought it might be a Mg problem, but I'm not seeing an leaf curl. I also thought purhaps a K problem but I'm not seeing leaf yellowing either.
hbbusa28 Reviewed by hbbusa28 on . Brown/copper spots on lower leaves First time grower needs some help. My plants are about 2-3 weeks old and recently I've noticed some brown/copper spots on the lower leaves. They started near the middle of the leaf in between the veins as a single brown/copper spot and started spreading though the rest of the leaf. Now on one of the most effected leaves some of the brown/copper spots are turning yellow/white (but that color isn't spreading through the leaf it is just the spots). I have two plants (one of them didn't Rating: 5