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 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.

Yea man i cant really help you out on the spots but my plants r doin the exact same thing and id love to know wat the hell is going on there cuz they were growing just perfect untill 3 days ago n now there is brown spots everywhere.Im flushin the plants with ph correct water wit nutes hopin its a nute burn but notin has changed yet.

So it's been a few days now. After doing some more investigating I thought it might be an MG problem. The leaves started to curl up a bit. I bought some Cal-Mag and used 50% strength and sprayed in on the leaves. Didn't help. The problem continues.

I read some more and thought purhaps I mis diagnosed and maybe it was actual a K problem. So, once again I sprayed the leaves with a higher K and still nothing.

It's getting really bad. The problem starts as small brown/copper dots in between the veins. It then quickly progresses into larger brown/copper areas. Eventually taking over the entire leaf. It started on the lower leaves and has been makeing its way up the plant. After a few days the leaves turn all brown with just a hint of green patches, brittle to the touch (in fact with I touched the leaf it crumbled)

Please help with any suggestions. I'm using GH 3 part formula in a hydro drip system. Currently I'm using a (18-8-24) formula (that's 3-parts Grow, 2 parts micro, and 1 part flora). 400mh lamp about 20 inches away from the tops of the plants. My PH was at around 5.9-6.0 but after reading that K is absorbed at around a 5.2-5.6 I lowered my PH to around 5.4-5.5 hoping that it would absorb the K.

I have also flushed the system to rid it of an salts that might be locking up the rockwool. Still no luck. I'm affraid they are dying. Please take a look at the pics and let me know what you think. The one on the left has always given me some problems, you can see it is a little stunted. The one on the right was doing good until recently. Now it is becoming like the one on the left.

Please help!

I had the same problem with my plants about a week ago but i figured it was a nute burn so i lowered the nutes and my plant went back to normal and is growing just fine again so not sayin it is but it could possibly be nute burn.

Yeah I don't think this is nute burn. My PPM is about 900. PH is now at 5.5-5.6. Still no luck. If it was nute burn wouldn't the damage start at the margins and work its way back. This isn't like that. It started in between the veins as single brown/copper spots then spread like the dickens. Taking over the entire leaf and moves upward.

yea mine did the exact same thing started at the veins and spread on thw whole leaf but my ph was just fine to around 6 6.5 but i added to many nutes so mine was nute burn urs could a be a k problem or maybe evenm a mg problem.

Its nute burn I would bet my left nut on it your plants are small they should only be at about 200 ppm at that stage with that light I run 2k when my plants are that big I am at only 400 ppm plus it is strain dependant but I know for a fact its nute burn.

Thats what im thinkin it is to cuz mien was fine after i flushed em n added less nutes.

Thanks guys I'll give that a try. But recently it started perking up when I adjusted the PH. My PH was 6.0 in the res but when I check the run off it was higher. So I flushed the rockwool with 5.6-5.7 PH until both the res and the run off was the same.

Now the plant has picked up a bit and resumed growing. The older leaves are still F'd up but the new growth is looking good.

New pictures.

yea man there lookin pretty good i have a feelin there gonna be some nice plants hopefully females but umm yea about ph is it spose to be around 5.5 cuz mine are constantly round 6.5 n they r doing great.But ur plants look awesome keep it up.