Stun grow, yellow leaves, is this Nitrogen Deficiency?

[lincon1231]

Can anyone tell whats the problem here?
It's pineapple express auto grow in coco

Cuz the seeds were exploded to lights for years, so have a really low germinate rate and slow grow in the beginning
Always water in ph 5.5-6 under 600w lights, humidity always around 60%-90%
so i think this can only be nutrient problems

in the first 2 weeks seedling dose not grow at all but the root process is still good.
now she's in week 4 and now the lower leaves turns yellow.
I think i over water them a bit, and the last feed was in around 600 ppm with FF nutrient and Cal-mag

Im staring a new grow with fresh seeds and now they look good.
just wanna figure out what i did wrong and won't make it again this time’

 

[Waltermelon]

Personally I think she's devouring herself to transport food from the lower old growth to make new growth. I look at it (I have been wrong before.)as a survival method. It's killing the ENTIRE leafs on the lower growth in hopes it can find a nutrient store in the soil sometime in the near future and sacrifice the nonproductive lower part of it's self for a more productive top.