I have read about people keeping the upside-down jelly fish. the biggest thing that you have to watch for is to keep enough current around the edges of the glass that the jelly doesn't touch it. I don't remember exactly why, but I thought it had something to do with them collapsing from the impact.
We got them in at Pet Expo here and there. Always died. Very dissapointing to see them even come in as you just knew what was gonna happen. Same with Octopus.....always came in no one bought them, then KACK. Although one time we got a blue ring in by mistake and someone did buy that. I hope they read up on that as I was not there when it sold.