What is that?

Calcium low or maybe too much potassium?

Thank you for your help! I regularly check irrigation water for pH. It's always around 6.5 Or do you mean if I check the growing medium itself for pH because I don't do that. I just watered them with bloom fertilizer for the first time. It might be that they are running low on P, K or Calcium since I am reusing the soil from the last grow without amending it apart from beneficial fungi & bacteria. Another suspicious finding is that only the ones stretching like hell are having those blotches plus I am going with medium strength fertiliser chart from biocanna which is calibrated for first use of Bio Terra Plus soil. But I made this connection after mixing irrigation water and feeding it to them. Otherwise I would have fed them a higher dose of BioFlores. What do you think?

Mmm what about pH? Are you checking it?