After reading your post, I have to agree with the other guys; we need more info on how you're trying to do this in order to see what is going wrong. Chances are it's too much water and too little light. It could be Nitrogen deficiency too. Do you have pics you could post? Also, are you doing a soil grow or is this hydroponic? We're here to help but you've gotta work with us so we can help you.
