I don't know anything about that soil. I googled it and apparently they have numerous different types. But nutes are always needed, regardless. I use Fox Farms Ocean Forest. Some people claim it's too "hot," yet I start my seeds in it and then start supplemental fertilizers when the plants are about 1 1/2 weeks from seed.
Your plants are tiny for 3 weeks and are very pale, like they are lacking nitrogen. Here is a shot of one of my plants at 3 weeks. They are in 5 gallon pots for reference.
I'm not sure what to tell you. How is the ph of the water going in? And the runoff? What kind of lighting? I would definitely start them on veg food high in nitrogen at the very least.