Auto Whiteberry

Depends on how "charged" is your soil; with a light mix you'll want to start after the 2nd week and go easy, probably around 0,4 EC (200 PPM) once per week, so to see how the plant like that strength (leaves turn dark green? Go with less or none at all, next time. Leaves turn light green? Go with a little more, like 20% more. With 11 liters of heavy mix soil (3 gallon) I don't have to fertilize until after 8 weeks, except for a calcium / magnesium additive. When you start plants in a big pot from seed, they usually develop slowly, one of the reasons for this is because it is so easy to over water, which makes the roots want for oxygen. I see that you are giving them 500 cc every day? That's way too much for a seedling