If you are using fertilizer now, it's too soon. Plants don't need fertilizer as long as they still have their cotyledons, as the cotyledons feed the plant. Wait until the plants have at least 4 sets of true leaves. Also, young plants don't need much - use at 1/4 strength to start with, when you do use them again. Note that if you're not adding ferts now, it may be the packaged soil has it added in. If this is the case, you might want to repot with starter mix. Either way, use plain water for a while to flush things out.