Nitrogen deficiency and reddish veins

Hello guys since a week my plant won't grow and I can notice yellowing leaves at the bottom. I'm currently using plagron alga grow at max rate (4ml/L) to treat the problem and I was wondering if they was anything else that I could do beside waiting.
Also the veins looks quite reddish to me. Is it normal?

Soil...ph ...lights using? More info the better. Water schedule.

It does look hungry.

I'm currently using biobizz light-mix. I never checked ph level because I'm using organic nutrients. This plant is under 250W HPS and is 3 weeks old.

Concerning water I don't really follow a schedule. Usually I do it every 4 days but last tuesday when I watered my plants this one start dropping straight. I presume that I was overwatering my plant so I potted it in new dry soil with a bit of water for the root system. 5 days after I watered the plant and leaves rose again. That when I noticed that cotyledons and some leaves started to turn yellow.