Looks like you need a higher level of nitrogen in your soil, but wait! if your not using quality Nutrients you will want to wait 3-4 week before doing so for 2 reasons
1. the plants are still young and you could kill your plants through Nute burn.
2. your premix soil already contains a cirtain...
The position of your light could make your young one lean, make sure your light is directly above your plant & as close as possible without doing damage, this should correct your problem.