I'll second the cal/mag.. I think in mentioned it before.... If you are still using that purified water, it likely doesnt have any minerals in it like you would find in tap water... People who run RO also have a bit of trouble with cal/mag, it is pretty much the one thing in hydroponic nutrients that is a bit lacking, partially because the calcium negatively interacts with other nutrients in concentrated bottles... I use Technaflora Magical, in every other water change.
Also, don't up your nutrients (PPM) at all because of this, it will make matters worse. Just get some cal/mag and keep it as part of your routine. Even if you end up using tap water, during flower you will still end up needing some supplemental Cal/Mag..... Another good one to use periodically is silica, it helps harden up plants and make them more resistant to pest and other stress that we put these plants through..
For now, leave the leaves that are affected - on the plant. The plant will strip that leaf or leaves of anything it can use. Then remove it when it looks dead enough to...