LUZCA8

@HighBigFella, Thank you!

@QuickBricks, Thank you very very much for your thoughtful and long comment, I do appreciate it a lot! In the meanwhile I found out that the landlord installed a decalcification system, in this case it actually also could be that there is too little CalMag in the water due to the decalcification system. I have started giving the plants 0.5ml of BioBizz CalMag. I usually pay attention on how I water, I water evenly around the pot with 250ml and then when its soaked up another circle of 250ml. Once again, thank you! I hope the plants will be fine and that it is indeed a CalMag deficency.