Need some expert advice on leaves

So I have 6 plants. Three gelato auto and three northern lights auto. Both strains and seeds were from different growers. So all were germinated the same. I'm starting week 3 and two of my NL are looking like they will not make it. Ones is just small but still looks ok. Just a little tip yellow on the leaves. The other has been droppy for a week and now is getting yellow also. The third looks ok but not as big as my gelatos. My lights are on 20/4. I haven't watered in a week. Because I suspected the down curled leaves on the one was a sign of over watering. My lights are a 1500w led almost 30" away. So thought maybe the yellow might be from light burn, because I haven't give the plants any nuts, and they are in new homemade soil, so they should have plenty for now. Anyone have any ideas. You think this is light burn on the NL even though the gelatoes are doing great or are my NL from on grower just bad phenotype? Also temp is 78 day and 68 night at 70% humidity. Air exchange is every 10 min. First three are the NL pics and the last three are the gelato.