A gentle fan moving the air around the plants will strengthen the stocks.
What temperature (color) of light are you using?

Seedlings in grow rocks already is not really a good idea. If you had rooted them (even from seed) until you had 4 sets of true leafs (then you are in the vegetation cycle) at that point then you have a true plant. The stems will always stretch a bit and you can use that when you transplant them to get the roots deep in the net pot the put the rocks in on top. That will hold up the thin stocks and give the plant a good base to grow from and provide strong support for later on in it’s life cycle.