stem problem / questions

i've recently sprouted some random seeds i'v had laying around and a bunch of them got about 2 - 3 inches tall before they really started showing vegitative growth. im on my 3rd set of leaves including the small round ones. the issue im having is that the stems dont seem to be thickening up. a few sprouts have layed down beause the stem can support the 6-8 sets of leaves that have formed. i'e had a fan blowing on em since the get go but it doesn't seem to be helping.

basicly im owndering if there is any nuitrients or anything i can do to toughen up the stems.

any input would be apreciated

I would just stick with a rotating fan and if it starts to bend put a stake on the plant to help it stand

Get them as close to the light as you can with out heat stress. There stretching for the light.