That medium looks soiless, is it? What is the PH?
I don't think it is over fertz, but that is just me. Your leaves are too yellow and pale looking for that. Due to young age, I'd rule out salt build up/lock out, unless you are recycling soil...
can you provide some more info? Type of lights, how often you water, soure of water, PH of water, nutrients, how many, how often?