Hey there Cannadisco, I cant see it 100% but it could be one of two things, probably the second thing. First thing that you should always check : it could be pests, specifically it could be spider mites. Check under your leafs and look for absolutely microscopic little black dots ... if you find some of those, just write me a message I'll walk you through it. It could also be calcium deficiency, that's more likely. You can tell it's calcium because of the small rust colored spots that affect the older leafs first. To remedy this : nothing simpler - add "calmag" every other watering or so for two weeks and the problem should stop spreading and the plant should recover within 10 days to a fortnight. To cure you plant quicker you can also make a calmag foliar spray with 3ml/L and put the solution into a spray bottle. spray just before lights on every three days in addition to what you put in the feed. It speeds things up some. Hope this helps ! đ