Possible Deficiency, Leaves curled upwards, spots?

[murph420]

Hi everyone !

(NoOb here). First time trying Coco grow and I just recently transferred rooted clones into plastic cups. It has been about 2 week and a couple of the leaves have started to curl upwards a little bit. On top of it a couple of yellowish and brownish spots on the leaves have showed up. Stems also have purplish tint to them. I have been feeding them every couple of days at 50% of the recommended dose off GH flora feeding schedule. 2 out of the 5 clones only have the issues.

Details:

Strain: Girl Scout Cookies
Medium:botanicare coco coir : premixed with 200ppm of Calmag ph at 5.8
Nutes: GH 3 part. Week2 of Veg schedule feeding at 50% of recommended dose.
Ppm : 300
Ph : 5.8 (runoff also at 5.8-5.9)
Water : RO
temps and RH : 70-74 deg / 47-51%
Lights : T-5 4 bulb
Fan/exhaust : 6 inch exhaust with filter and ocilating fAn
Room: 4x8 sunhut

Pics:

 

[blackhole]

looks like light burn and nut burn I water only for first apx 3 weeks
you have holes in the bottom of them cups

 

[murph420]

Yes I have holes in the bottom of the cups for drainage.

The roots have grown all threw the cup, should I only feed them water still?

What can I do about the light burn?

After doing some reading I thought maybe it could be a magnesium deficiency .
Any input?

 

[murph420]

My light is hanging 18" + away from the tops of the plants to.

 

[murph420]

Can anybody give me insight on this problem?