Yellowing of Fan leaves on sides PLEASE HELP!!

i have a couple plants of what i think is a strain called bogglegum (master Kush x bubblegum). here are the pics, could it be nute burn? maybe lack of Mg or Fe. i cant figure it out this is my first indoor grow and i got 5 weeks left i wold hate to see these things die at this point. PLEASE HELP

severe magnesium def

a bit of advice, next grow start supplementing with calmag right when you flip your lights to 12/12 and you'll never run into this problem again.

thanks man, any other advice anybody?