black to dark brown leaf tips, are forming on the lower leaves, and have a very sharp upward hook at the tip. The tips of the affected leaves are crispy.
Could be alot of things going on there. Over water over nutes root bound, Post a few pics and check the rest of the grow guide. lots of pics and lots of help
definitely nute burn. those are too young to feed, or at least feed them very weak mix. flush them and let them dry out. do another watering with plain water, until they are growing again. After you see some growth, try doing a 1/3 mix of the nutes. Don't overwater