OG Cali Haze - 1st grow, issues early

Hi! I'm looking for some help identifying problems that I'm having with my three plants. They started having brown spots on the leaves straight away.

I'm growing them in 5gal buckets with Promix all purpose, some compost and some lime. I started this mix on the recommendation of a friend but I've since discovered that it's definitely not ideal. I'll switch it up for the next time.

It's been 20 days since I planted the sprouted seeds. They are drooping and dark in colour. I'm thinking it's nitrogen toxicity? One is also growing very asymmetrically.

I've been feeding with tap water that I've let sit for 24h+ and adjusted the ph to around 6. They're on a 24h light phase now. If I'm leaving out information please let me know.

Looks a bit too hot and locking things out, i personaly would use my hands to get deep under the roots and try to transplant to a new (lighter) mix, or feed less.
Its a bit hard because you have a small plant in a large saturated pot. If you flushed you could do more harm than good because the pot would take so long to dry out.
If you can, put it in some lighter mix, or try to mix a bit of coco in yourself by turning over the soil (being careful not to damage roots), but if possible transplant.

Thanks! I’ll try to get some coco in there to drain it out better. Promix definitely retains water well.