-No ferts at all to start
-Unless you are going into hydro, and even then, rockwool may not be the best choice for seeds. I find it difficult to work with, and prefer unferted soilless mix or rapidrooters for seedlings.

How large are the cubes? If you started in the little plugs or 1 1/2" cubes (coorect size for seeds), you should definitely see roots by now.