Black spots under leaves

High Bro, The cause for these black dots is a calcium deficiency. Instead of writing a long story about everything that could have caused it and the solutions possible I can better send you a link from the growweedeasy website with an article about everything you need to know about solving and preventing this deficiency. This article contains pictures that make it extra clear and you will see that the calcium deficiency is the problem because you can compare the poctures with your own plants. Here is the link: https://www.growweedeasy.com/cannabis-plant-problems/calcium-deficiency Enjou your grow!