I wish I had a copy paste directory for questions like this. Relax good sir, your plants are likely fine. This is a common question believe it or not. If one is doing fine and another is not but they have received the identical treatment, its plant genetics old boy. I've got two right now that I started at the same time. For awhile, #1 dwarfed #2, now #2 is catching up but still smaller. Why? #2 has a weaker genetic code than #1. If you've watched the ready, set, grow videos released by high times mag they'll even tell you "Some plants are healthier and more vigorous than others" If you've got one plant thats just beating all the others to a pulp, you've found your mother by superior genetics. This is how botanists improve crop harvests, make plants resistant to disease, etc. They find the beefy guy.
Basically this is a long winded way of saying; You've got one healthy seedling, you're doing things properly. Keep on keeping on and don't try to fix something thats not broken. Overreaction is the leading cause of death in plants that are being tended to by new growers. Weed IS a weed, it'll do fine.
Best of luck, check back with how they're doing in a few weeks?
