good lighting. and keeping light close without burning. seems like the further the light is away from the plant the more the plant stretches the closer the light is the fuller and less spacing between nodes
dont have the light to far away from top of plant after the first week of sproting cut a 5 litter bottle in half and stik it ocer ur plant then u can have the light as close as u want take a look at my pics
instead goin and buying another whole light system why dont you buya mh conversion bulb that runs off a 600 watt hps ballast, the light should be about 18 inches off the plant
a good rule for lighting, If you can hold your hand on top of the plant without burning your hand - it's going to be ok for your plant too. The closer the better.