I'm not familiar with Build A Soil 3.0 but it looks like it's supposed to be a living soil that is 'water only'.
You mention that you're pH-ing your water? (or is it just naturally around 6.6/6.7?) If you're pH-ing, you can stop. 99% of the time, organic grows do not need the water pH'd. And if you do adjust the pH, make sure to use organic ingredients (ie. lemon juice, vinegar, etc). The chemicals in pH-up and -down will kill your soil microbiome and will lead to trouble. If you've been using chemical pH-up or -down, stop doing that and just use regular water out of your tap. The fungi and bacteria will do the rest.
If you're not pH-ing your water, are you using synthetic nutrients? If you are, see above - you can stop. Synthetic nutrients are salt-based and they kill the microbiome in the soil. Plus, with living soil/organic grows, you'll add amendments, not nutrients. The fungi and bacteria break down the amendments and take care of the plant.
Seeing a pattern? In organic grows, you take care of the soil's microbiome and the microbiome takes care of the plant. It's like magic, but it's not...it's nature
What is your watering regime like? Not knowing how old your plant it, it's tough to give a good recommendation but I will say it's easy to overwater soil, especially organic soil...it will soak up excess water like a sponge which isn't always a good thing. It looks like you're probably a few weeks from seed. If this is the case, you should be giving 3-4 cups of water every 3 days or so. If you're overwatering guess what it can do? If you said it can f-up the microbiome, you're right! Not only that, but it can suffocate the roots and promote pests and disease.
Looking at the leaves, and with the information that we have, I'm guessing it's a watering issue. Let us know how much and how often you're watering, and maybe it's not a watering issue but we can go from there.
Whatever you do, you should be proud of asking the right questions and doing your research!
