yellowish leaves and small brown spots

Hi I would like to show you some of my plants. One of them 10 days old other is more than 3 weeks. I had a nut deficiency isssue due to water ph and now I think they started to grow again. But with 10 days old one is completely a fresh start. All advices are well received. Thank you on advance.

Btw old one turned little yellow after fixing ph problem of water and gave fertilizer. And I m making 12/12 from seedling so I took the photos at dark period so the leaves curled down.

3 rd photo stem colored purple

Ph of soil - 6.75
ph of water i gave - 6.0- 6.5
light 600w hps
distance from light min 54 cm for new seedlings stay at side approx 60-70 cm away from light
temp at room - 27

Did you do anything to your soil? I noticed in pic 4 the soil looks really packed down? What nutes are you using and at what PPM? JAS

biobizz bio grow I gave them 1ml for 1 lt which says on discription

Try to keep your water ph between 6.5 to 6.8. Also, you might want to add some finely ground dolomite lime to your soil in the future to stabilize ph.
Imo, I use biobizz me self and it is da bomb. I dont consider the bio-grow as a stand-alone nutrient. Add alg-a-mic, bio heaven and root juice for the micro-nutrients and micro-life.

You need looser soil, I used to use this shit that when it got wet it compacted a lot and it caused my plants to get burns in their leaves. Go to the store and make a mix of half whatever soil you like and half sphagnum peat moss dirt, it holds moisture great and allows your roots to grow as needed. Hope this helps