They're streching trying to reach the light, but not getting enough of the energy to actually grow very well. I just started up again after about 10yrs, and had forgotten all my techniques. I got some bad advise, and after I first germinated a few, I kept the lights too far away (was told to let them get used to the world before blasting them with any light) so they used all the energy contained within the seed to stretch as far as they could towards the light, then 4 of 8 spontaneously died (Im assuming they were just exhausted trying to reach the light and were just spent) and the remaining 4 were stunted and never fully recovered, after a month they were still spindly and I had to kill em off to make room. Since then Ive been starting my seeds into rockwool, once the seed is in the hole I gently pinch the rockwool closed over it so it can push through and sprout just like in nature. And I give em full strength 6500k cfl light at about 3" right off the bat. They push through short and fat, with thick strong stems and stay squatty and bushy, about half as tall as your seedlings with 4 internodes already. The proper temp light (6500k) and strong intensity will promote short bushy growth, HPS temp (2100k) will encourage stretching. I understand financial restraints, but a few 26w cfl's will do much better than your 400w HPS simply due to the color temp. Your 400w will kick ass once they've been able to veg for a couple weeks. But for now, they're babies and you gotta baby em, I'd reccomend a few $7 GE 26w cfl's for now, and make a reflector for em with cardboard and aluminum foil, or a reflector w socket are $7 at home depot. The nute advise is good, use 1/4 strength until you can see visible growth and new leaves developing, then slowly increase the strength until they're established. If youre using a bubbler you wont have to worry about overwatering, overwatering is simply oxygen deprivation. If you have sufficient air bubbles in your solution your roots cant drown, eventually your roots will grow into the solution and be constantly underwater, but the bubbles will let them breathe. I see you're in rockwool plugs with hydroton surrounding. Fill your reservoir until the water level is about midway up the plug, and let it get down to the very bottom of the plugs or a little below before refilling. Hydroton doesnt wick water very well so if the level is below your rockwool, it can dry out (will take a while though because the inside of your bubbler is very humid and the rockwool will absorb moisture from the air inside for awhile)
My plants are top fed via feeding tubes pumping water directly into the rockwool cubes so no need to fill the rez that much. As you can see here:
