First off, that pot is way to big for that little plant, get it into a smaller one, if you dont, the roots will grow huge, the plant will stay small, and re-potting will be a bitch... like.. you will have to find a huge pot next re-potting, just for a small plant.. which wastes space...
That brown stuff on the leaves is either heat stress, mg deff. or light nute burn, so if i were you i'd flush any remaining nutes out of the pot and add some epsom salts.. if it continues to burn, either move the light away, or add some ventilation, and get a little fan blowing across the light.. just so temps should drop a little.. (make sure the fan goes off at night)

hope that helps... i think it mostly looks like heat stress and over-watering, but i cant be certain... - good luck...