Okay, a few thoughts in the event it's not mites...
If you don't find any mites, you might have a calcium deficiency. Are you confident in your PH? What are you using for water - RO filtered, tap, distilled, etc.? I've been lucky enough to avoid mites, but based on pics I've seen Jandre's suggestion seems like a good one. I have seen calcium deficiencies that look like the top picture quite a bit.
If no mites, consider doing a flush of your medium to be safe (if you haven't or haven't been). If using RO water and no Cal/Mag, perhaps add cal/mag to your regimen.
My preference for mid-grow flushes is to give them a feeding of 1/4 strength (or so) nutrients. "Like" molecules bond to "like" molecules, so believe it or not a 1/4 strength feeding actually is more effective at removing salt build-ups in the medium than straight water is. I like to pour through 2-3 times the pot size of 1/4 strength nutrient water when I do these flushes. These "diluted flushes" are pretty easy on the plants, too and not as shocking to them. In my experience, they usually look really happy to have their pipes cleaned, so to speak. Then, make sure you have sufficient calcium in your nutrients or water, or consider adding the cal/mag.
Make sure that you have an accurate PH meter, too. It could be PH reasons that are causing the deficiencies, and sometimes PH problems can cause leaves to look a little like these too. I like my PH in hydro to stay between 5.5 - 6.2. It's good to vary it also, so that maybe one feeding they are getting 5.5 and another 5.8 and another 6.0, for example. In active hydro systems you can let it drift from 5.5 to 6.2 and then adjust back down - so it's working through that range. PH meters need to be frequently calibrated and can get very off in short periods of time. I calibrate mine at least once a week (my new one seems to need even more frequent calibrations).
Are you measuring your PPMs? If so, do you check your run-off when you water? If the run-off is coming out considerably higher than the PPM's you are putting in, you may have salts built up in the medium - warranting a diluted feeding flush.
This is all assuming it's not spider mites, which it may be. A sponsor here has a product that is supposed to be pretty good for mites: Sierra Natural Sciences SNS-217. Also, Hot Shot "No Pests" strips are supposed to be effective mite deterrents and can be hung in the room near the plants. Supposed to be safe for humans to be around up to 3 hours per day, but read up on it to make sure you are comfortable with the vapor they emit.
Really hope you get it figured out and that it's not mites!