Have you checked your Ph levels before feeding and then runoff PH? The same for PPM's/EC
This actually looks like something called black spot. I had it on an outdoor grow and its an outdoor problem. But with all things outdoor it could be introduced indoors. Have you brought anything in that has been outside? Pot, soil, really could be anything even you.
Is it spreading? How long has it been like this? And are there any other symptoms other than the spots?
Edit to make a point that this could be early phosphorus deficiency... check you nutrients make sure your giving the right amount etc.
Also what are you using for nutes?