I like to use basic nut-less soiless mixes like we used in commercial greenhouses. Pro-mix being my favorite (the standard). Its a light mix consisting of milled spaghnum & perlite. I'd try mixing something similar into what you already have. For future reference, keep in mind the root structure of a new seedling. They burn very easily. Its best to use a mix with no nutes, NO feeding untill the second true leaf, & then the first feeding is best if you use a very diluted flowering nute. Why? Because it will stimulate root production. That would be phosphors & potassium. Go to your favorite growth nute after that, then your baby will have a need for nitrogen.