Seedling stems grow long thin and weak if the light source is to far away from them. Id recomend a T5 flourescent system as the plants can touch the tubes and wont burn. During the first few weeks HPS is too strong for the plants anyway and you need to concentrate on getting those stems as short thick and strong as possible to support whats to come. I had the tubes an inch to two inches away from the tops with a low level fan which i increased as she got stronger and have really strong stems. Regarding space, topping, super cropping and tieing down will help. d0ct0r