One of the LFS
's I go to occasionally has cassiopeia sp, Upside Down jellies. They look pretty cool...I guess.
From WetWebMedia: Cassiopeia spp. are zooxanthellate organisms that require large, intensely lit, mature systems, with low flow, a deep sand bed, and available nanoplankton. Also of note is that these Scyphozoans shed stinging nematocysts into the water, which can potentially sting you and/or any other livestock therein. All things considered, these jellies do best in large (over 100g) species, or biotope systems.