It looks to be a nutrient problem. Are you adding any nutes?
When doing seeds or clones you need very little nutes till they get about 10 inches tall or so. The soil mix could possibly be too nitrogen rich. It seems to be pretty common in a lot of these potting soils. I have very good luck with Sunshine Mix 3 for cloning and seeds and then switch to Sunshine Mix 4 for flowering. The only thing I add during seeding and cloning is B12 anti stress vitamins.
killerweed420 Reviewed by killerweed420 on . HELP All seedlings die every time! INDOOR grow Hi,Ive tried to grow WW, silver haze and Northern lights a couple of times , and I fail every time. I always reach the stage of the seedling has first or second true leaves. I have tried many soil mix including Hydrponics but every time its the same they die. Im also growing tomatoes and peppers The peppers some times live and grow to maturity Attached are pics of my setup. Soil FaFard seedling and clone mix. Temp is always between 21~ 25C Light CFL 3x 32W RH 45%+ Water , RO PH Rating: 5