first of all, the light needs to be a lot closer to the top of the plant. basically as close as u can get it without burning the plant. if ur hand is just a lil warm when its right above the plant, it should be good. the plant is stretching because it wants more light. it thinks it is surrounded by other plants, and a lil bit of light is coming from above it. so it stretches up to that light, in order to get ABOVE the other plants, and thus recieve more light. if u move the bulb closer, it slows that process down.
second... i dont do hydro, but im pretty sure 1200ppm is too high for that small plant. usually 1200 is for flowering i believe. in the first pic i can see nute burn on the leaf, so maybe back off a lil on the nutes. do u know your ph?
third... a 600 watt light is way overkill for that one plant.
HERE is a link to my grow journal which i started yesterday. i have a 600w in my closet for 2 good sized plants, which is still overkill. i plan on getting more soon. what im saying is, if u only plan on growing one plant at a time, id go with like a 400w and make it one big spread out bush(look up LST). but if u plan on getting like 4 more plants, by all means go with the 600. some even have a switch where u can switch from 400w to 600w
also i suggest getting an oscillating fan to blow on that baby a lil bit. put the fan on low and have it spin. wind makes the plant grow thick and strong. if u were to flower that now, i dont think it would even hold itself up