Mate, does your tent have a white liner on the inside? If yes, you may have a 'toxic tent' Symptoms on plants are as follows: It starts with a MG deficiency which progresses to an overall nitrogen deficiency, after that new growth stunts and the bottom leafs start to bleach out and gradually start dying off. Easy test to see if it is your problem, take a plant out and see if it starts recovering with in about 7-10 days.
Oh yeah, I forgot to mention, nothing you do short of removing them from the space will help them. Root system will also be under developed since the plants can not process the light anymore once they start to get poisoned. I can't believe that no one that has posted in this thread brought this up as a possible cause?!
Unity