I'm 100% a newbie, so take this as just a guy talking only. If you look at my grow I use a "Layer Cake" approach, the seedling starts in M1 seed starter which has 0 nutrients and light pack within a 3" pot or solo cup volume, then around that it grows out into Happy Frog which is a 2gal pot worth, then once the roots pass that it hits Ocean Forrest which is the remainder of the 7gal pot. This approach, hopefully, allows the plant to grow and age into the nutrients in the soil. Unless the soil you bought is truly seed starter or labeled 0 nutrients, there is a high likelihood it does have nutrients in it. PH wise you're dead on where I like mine, 6.5-6.8 and temps are in the right range as well. I am only guessing, but check that soil, watch the next sets of leaves, if they do the same or start, maybe flush your soil really well since you're still in small pots and won't ruin a larger volume.