Before you add Hydroton, sprinkle some cinnamon around what you've got showing, making sure to get it on the stem. All that young stem is just asking for damping off.  Also, be GENTLE adding Hydroton. Don't wanna crush that stem.

 Next seed you pop, MORE LIGHT!!!!  And as soon as it stands upright, put a fan on it. These two things will help you develop shorter, stronger stems.