Now you went and put the plant under a hot light 24 hrs a day for 3 weeks. The discoloration is light burn from having the light to close, it's not magnesium deficiency. Don't water with this heavy a solution of Epsom salts it will lock out Calcium and really mess them up.
I see that you transplanted. Fill the pots up! 1/2" from the top. Then do like I told you and put the plants outside in full sun. If your lucky you didn't screw them up completely with the light 24-7 and they won't Hermaphrodite on you.
Let them dry out and wilt Pull the tops down and tie them down to the pot while it's wilted and soft. Then water.
Go to the hardware store and get a jug of Pro Mix 12-6-6 and a jug of 5-15-5 for flower.
You will need some
Cal-Mag 1/2 tsp pr gallon for the entire grow mix in the 12-6-6 1/2 strength for 30 days then full strength and feed the plant once a week. Once they are 4' tall and real bushy like this you may need to up the feed to twice a week but once should be fine.
Once it's stretching up the
cal-Mag to 1 tsp pr gallon and when it's done stretching switch to the 5-15-5 for flowering. It's just that simple.