Hi,
I posted a little while ago about my hygro developing holes and dropping leaves. This was put up to a sudden decrease in co2 which makes sense. But after three weeks the plants are still dropping leaves, and I finally got my nitrate test kit refill and found out that my nitrates are only about 5. This reading was taken the day after my weekly fertilization. Evidently I need to fertilize more, I slacked back to 1x/week since I dropped co2 but maybe that's not enough. Would this add to or perpetuate the problem? Or does it not really matter if nitrates are low, as long as you don't run out?

If your nitrates are close to bottoming out then it's likely that potassium is also low or has run out. Try adding more K, though increasing overall ferts doesn't hurt either.

I added a bunch of nitrate and within two days it was near 0 again. I guess I need to start adding more ferts, plus increasing the frequency. I'm increasing everything, since like you said, if one is running out probably others are too.