Hi there Leo, pleased to meet you ! the fertiliser was too strong for the small plants in the small pots so they are "locking out" nitrogen, that means they are not absorbing Nitrogen anymore and they start to yellow starting with the oldest leafs first. repotting could help, but the main thing is stop giving nutrients. the best thing is to start again with less nutrients. Sorry for the bad news my friend ! Good luck with your first grow ! π