HELP! brown spots and light green new growth?

3 week old skunk growing in 6" pot 1/2 miracle grow soil, 1/2 coco, 30% perlite and a little worm castings. i havnt added any nutrients but after watering i noticed brown spots, leaves began to canoe and new growth came in light green around the edges of the leaves..any help from advanced growers would be much appreciated!

I'm not an advanced grower by any means but I think this may be a Mg deficiency. Maybe some cal-mag is in order.

Check this site out, it may help: http://www.ganjaguerrilla.net/

I forget who posted this first, but all credit due to them, not me