
-- it's that damn Fuxfarm OF soil probably,.... Just to be sure, how far away are the lights?
.... OF is notoriously iffy soil mate, [HASHTAG]#1[/HASHTAG] problem soil I see here sad to say, despite it's unaccountable popularity... some get OK bags, some don't... Either way, it's a rich soil, too rich for many strain's seedlings,.. better to use something lighter, that way you can start to feed sooner, a more balanced complete spectrum of nutes vs. WTF ever is hucked into their schwag soil,... Usually it's badly off pH too, leads to a host of problems; you'll need a decent soil pH probe to check... the Accurate 8 is a good unit, mine has served me well!

hsoil: (long probe)
Symptoms look weird, like Fe or S defc, but with necrosis, ... very likely more than one thing is in play here though...
..wait, you say the plants are nearly 3 weeks old? If so, they are epically stunted, might not be worth trying to flog then back into shape... flowering should start soon, and at this size, ....

Something to consider,... And you do need some basic equipment, a pH meter (liquids only with them), a TDS/EC is handy,.. and that pH probe to really manage things properly, in-pot pH stability is key,.... No more

soils! ...Use at you own risk,...