Hang on!
Just saw your other post.
Your problem is most likely heat if you are using the HPS for seedlings.

That color temperature causes stretch.
And the heat produced is radiant.
The plants see it as IR and reflected "far red".
That demands stretch.

Shut off the HPS for now, seedlings do not need, and can not use, that many lumens, and they are spectrum sensitive as well.
A couple 23 Watt, cold white CFL are plenty and will fix your elongation.
Save the HPS for when you see the first preflowers.

Aloha,
W.
Weezard Reviewed by Weezard on . Stem problems on seedlings I've had great success germinating seeds. My problem is that many of my seedlings end up with stems the size of thin thread just laying on top of the dirt. Pam I overwatering? Ph too high? Rating: 5