Disclaimer: SOLELY my opinion based on my previous grows.
There is also some kind of micro nutrient problem, the leaves are 3d etched and wilting. It is weird they all seem to be having slightly different issues, do they share the same nutrient reservoir?
The wilting in this case means you need more O2 in the water. The PPM should never go over 1200 ppm and the CO2 bags, commit and go with a 20lb C02 cylinder, you will thank me later.
Being in water you do not get normally get a lot of time between problems and failure, but CO2 acts a respirator to an injured person and a steroid to health plants giving a slightly longer corrective period. You also consider Diamond Nectar (Humeric acid), a water chiller to keep the water at no more than 68F (more O2 absorption in colder water), an O2 4" bubbler stone at the bottom of EACH plant, 30% H20 (5ml/gal - every 3days) to keep roots healthy, white and free from algae. You will want your PH to drift between 5.8 to 5.9 (veg), Early flower 5.9-6.1, Late Flower 6.0-6.3. This allows the plants to absorb all of the various nutrients that can not all be absorbed just at 5.8 ppm. Another awesome stimulant is Super thrive in seedling and Veg mode. A Silica product (like Silica blast) would be very helpful for plant vigor and rigidity, plus more trichomes. A GH Koolbloom extra blooming agent is helpful to super charge to flowering. ALWAYS use liquids, never powders to keep salinity low.