U need more light.. ( i can tell by just looking at the plant...) its growing to accommodate the light.
Remember, plants dont grow just because there is light, they grow because they feel they need more leaves and branches to absorb it all. So for example you use 1 23WATT cfl, it iwll only grow enough leaves so it can absorb the light and in ur case, just a few.
You put a 600watt HPS, it will grow more, because the light is more powerful..
Ph your water..
Dont over/under water
THATS IT!