I think that they need to be kept in high-pressure systems or else they will in effect disintegrate. This is from what I have read.
I would venture a guess that one person (at a maximum) on this forum has kept jellyfish, but if I am right, you may never hear about it. Ask at a local public aquarium as to what goes into keeping them alive in exhibits.
Then, please post here, because I would be interested in learning the full story.