Seedlings need nothing but water for roughly the first month. Especially if the soil already comes prefertilized. Superthrive has killed many a seedling in the past, too. It says on the bottle that 1 drop per gallon is enough. They're not lying about that. Superthrive is some seriously concentrated stuff, and easy to overfeed with.

I'm willing to bet that those plants are rootbound. Repot into a larger container, and maybe think about watering with PLAIN water and a drop of Superthrive. After the plant has 4-5 nodes, then you can start fertilizing if you need to.