Looks like you're short on all 3 of the big boys, Nitrogen, Phosphorus and Potash. The leaves on the left are a perfect example of missing all three main elements. The leaves on the right are just Magnesium short.
Change your res and bump up your PPM SLOWLY. Keep going up a little every week and watch the plants. At some point you'll see the leaves start to curl under like a bird's claw. You may also see a small amount of tip burn on some of the strains. The color will get really dark green. Once you see these symptoms then you now know the high-point PPM your plants can take. The PPM you're running now is the low-point. Take the low point and high point and run your plants somewhere in the middle. I like to call this "dialing in your strain". Notice how I said strain and not plant? Every strain is different. Enjoy the trip!