At the beginning of this week the first real leaves are formed. From now on the growth should be visible and continuous.
Day 18: In the middle of the week the next pair of leaves is already growing.
Day 21: Last day of the week. I stabilized the 4th plant a bit because otherwise it would have fallen over. I also rinsed it today and found that the ec value was about 2.1. Then I rinsed the soil to get an ec value of about 1.1.