Yellowish leave tips (Pics) HELP

Hi guys , need some help asap. There seems to be a problem with my plants. I noticed several days ago that one of my two plants has a problem on its middle and lower leaves. It started with the tips of the leaves getting yellow and it expanded but not very much. It is definitely not a heat problem.

I grow 2 feminized brainstorm seeds inside a 0,8 x 0.5 x 1.80 (meters) closet with an 400W HPS lamp with good ventilation . I use biological soil (ALL-MIX BIOBIZZ) and the plants are 1 month old. I started feeding them after the first week with a light mix of nuts (called root juice) and a week ago switched to a stronger NPK solution ( Bio-Grow (8-2-6) with some supplementary nuts (called bio-heaven) which has some minor elements in small quantities.

The plant that basicaly has the problem is also the one that is bigger.

EDITED (8-2-6). Is it a good ratio? Could it be caused by the ratio of the nuts?
EDITED soil type

thnx for the responses.

wouldnt worry to much unless it spreads... could be nute lock or the beginning of nute burn depending on the soil used.. ph.. all that... but like i said.. i wouldnt worry too much unless growth slows/stops or it starts spreading all over... most plants get a few yellow leaves during growth anyways... not every plant will be 100 percent green through the entire grow...