what water are you using? if it is tap water you should test the ppm. if you dont have a tester, you should be able to get an analysis from your water company. or you can skip that step and use RO water or at the very least run your water through a brita filter or something. it is hard to pin point what might be the exact problem but if you get the basics down (light, water, air) the plants should bounce back. in my experience it is easier to just change everything so it is all dialed in than guessing what your problem is changing one thing at a time. ventilation is not only for heat exhaust and temp control but is essential for giving the plants fresh air with fresh co2.