CBD_W1ZARD

@CBD_W1ZARD, OH!! haha yes that could very well be it! I personally use filtered water and then add all needed nutrients and have been lucky to keep PH well and nutrients well. Okay well let's hope this clears any potential issue up as they are looking BEAST MODE in the making for sure! Cheers

@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.

Amazing initial growth! looks like those little plants are loving what you are doing for them so far. Hope your grow is super successful! Peace * I noticed your concern with the 'rust' spots on the leaves. I am assuming that it is the lower right plant in the photo? If it is this plant I can see what you are talking about. To me (and this is just to me) it does not look like a nutrition issue, yet. It very well could be simple uneven watering inside your pots. Sometimes the soil inside the container remains dry in certain spots through out the grow and literally never gets water if the pots are never fully soaked. This can cause the drying of leaves on one side of a plant due to a small section of roots that are left dry frequently that you may be unaware of. This is my thoughts initially based on your photos, and the health of the rest of the plant and other plants. Also the dry looking leaves are on the outside perimeter of your group of plants. I would try 'over watering' this specific plant on the next watering. Do not water early, just water HEAVY and really let the water run through the pot and ensure all soil is soaked. Wait 1 week or so and see if no more spots are forming. I hope this can help, didn't mean to write a novel. I just think it is more minor than you may be thinking. Cheers!