If you speak about DWC systems the answer is not at all, because the water is full of oxygen, because of the air stone. If you cut the air flow, they will drown, for sure. That's it. In one of my buckets, the air stone get out off the tube, and now it's giving big bubbles with a very funny noise. But it's still OK, the roots are very big now and will soon fill the bucket, drink 2 liters a day
Fhydro