Lighting and Growth

I am using a coralife 48in 6700K 4X65watt compact fluorescent fixture over 8 plants. This is my first indoor grow but I have grown many outdoors and at this age they were alot taller. The plants are 8 weks old and are about 4 to 7in tall, they are the densest plants I have ever grown, but they are short, the stems are outrageously thick. Why so short? Is it my light? I am using a hydro setup with GH nutes, they are on an 20/4 light cycle, and have CO2 supplementation. I'd love to have the density of growth but the height just isn't there. I also have 10000k and 20000k actinic lights, used 4 my saltwater aquariums, they too are 4X65 watt 48 in compact fluorescents. How would they do? I do have a 400w HPS I was going to use for flowering when they get taller, would using one of the compacts with the HPS hurt anything? Any info would be greatly appreciated.

possibly u have the lights a little too close trying back them off forcing the plant to stretch but i would like to say a small bushy plant can achieve the same yield as a tall plant if pruned properly."it's not the height that matters its the yield"

Leave your light alone. Prune the lower branches that aren't getting light. Sounds like an excellent SOG (Sea Of Green) set up.