The aquarium's exhibit is not quite as simple as it seems, but then neither are the jellyfish. They live in special tanks, that are designed to simulate the moving ocean, which keeps the jellies afloat. Without moving water, jellyfish will sink to the bottom and die.
Another challenge to keeping them alive is keeping bubbles out of the tanks. Bubbles can literally eat their stomachs out, or collect on the jellyfish making them so buoyant they also die.
Moving them from tank-to-tank is also tricky, because these primitive creatures are 95 percent water.
These tanks are at most large aquariums and extremely expensive and complicated. Though I have seen one in a LFS thats rotate the jellyfish in one big circle from top to bottom, though they stopped selling fish due to people thinking that their home set up will be fine just to have the jellyfish die.
