Help! Plants yellowing (pics)

My plant is about 4 months old, it was "stunted at birth". It's flowered but I'm keeping it on the 18/6 cycle so it gets bigger because it's only 14.5" tall. I'm using a 400w HPS lamp and the leaves seem to turn a light green color. The soil is Miracle Grow seed start plating mix. Any suggestions would be great!

Could be their dying due to not wanting to re-veg that happens...Umm I'm pretty sure miracle grow soil people say not to use, but I could be wrong about that. Other than that make sure your pHing the water you put in there. Also even if it's 14.5 inch's tall you can still flower it cause that don't mean you wont get a lot of bud. You just have to top it and keep it well keep up with, if you top it it will get bushier cause it will make it want to sprout more limbs. Well thats all the info I can give ya since I'm still new to this also, but hope I could of been of help.

Im not really sure whats best to do but id prob switch to 12/12 and see what you can salvage from it!

Goodluck