Day 44, 43cm, 45cm: about 2/3 of the size I expected. Most probably pH problem. The pH of my water tank is 7,3 pH, the one of the earth is at least 7 pH. That's too much, and surely the basis for all my problems in the last month. I will probably get some destilled water and mix it with a little tap water to get something about 6,2 pH. (I will do some tests with pH down, but I suppose the pH will go up again after a few days)