Some aquaria (reefs included) do have jellies, but likely not the kind you'd expect. Hydrozoans (hydroids) have a polyp stage and a hydomedusa stage--so with some colonial hydroid species, you'll get a literal, swimming medusa. True scyphozoan jellyfish don't make it into the hobby as often, but some people do have the polyp stage that later buds into true medusae. The hydromedusae will often survive, some being adhesive and attaching to the glass. The scyphozoan medusae usually do not.