If you dont wanna burn em mate dont feed in veg, just repot when theyre hungry, it looks like its lacking MG an N, but you say youve been adding Mg, in what form mate? and how often? Too much Mg can lockout other nutrients in the compo, but three weeks into 12/12 is the classic time for Mg deficiencies to show, i get it on nearly every grow around that time, after the stretch when the buds are starting top form.
And what evaporates the chlorine on contact? lol
I didnt know about the too much Mg locking out nutrients its a treatment for the Aquarium safe for fish & plants I use every watering then feed every 5th watering (10 days) this plant has been in veg for way too long something like 100 days plus thats why it was fed no room in flower room lol a mate gave me the cutting