Cal Mag only in 15 l pots/ 5l going strong

It could be CalMag, but I'm not entirely sure; it could also be a slight phosphorus deficiency. Do you have your pH level under control? And how's the EC looking? Both values should be between 6 and 6.5 for pH in the current phase of your plant, and the EC value should be under 2 to rule out any blockage/overdose. Regarding the picture of the White Widow, it might be due to the photo itself, but if it really is that dark green, it looks like too much nitrogen to me. But you say it's fine. Keep observing or upload a few more pictures with the issues to the diary. Hope you can make use of the info.