On my second grow ( a closet grow ) I had air bieng vented in and out of the closet. The closet however was in a room that was not vented. So fundamentally I was circulating the same air round and round. I had a problem that looks a lot like yours and it turned out to be leaf spot fungus. I tried various things to tackle it but nothing killed it... only slowed it down. I posted pics on here so Im gonna try and find them so you can see. I also had fans blowing onto the plants.

What Im trying to say is that fans blowing on your plants will not ensure they dont get attacked by fungus. A constant flow of FRESH air bieng brought in from another room or even better, from outside AND air bieng vented out of your grow area is a much better way of keeping it at bay. It's also much better for your plants development.