I saw some in an aquarium in South England last year that was about 1metre by 1 metre. The guide post next to the tank said they needed circular flow to keep in capitivity (from memory) but it just looked like they were in a washing machine to me!!
Some kept spinning and rolling round without being able to swim properly didnt look like fun from where I was.
What are you doin...buildin a breeder tank for a giant squid?!?!?
Cass seem to do better in a fairly shallow tank with little water movement from what ive experienced as they have a similar symbiotic algae similar to that of hard corals and as such have to photosynthesize. They are called upside-down jellyfish so that the tentacles will have maximum exposure to the light. Therefore they will also need quite a lot of light... unfortunatly they do much much better in a dirty tank. I've got the irritating situation of having millions of polyps in my tank and about 30 ephyrae (babies) blibbing around my tank at any one time. If you have them feed them a mix of coral food, metal halide lighting and newly hatched artemia.
For pelagic jellies you'd definatly need a krysal tank however which is the washing machine design which Scouse described.
Because jellyfish are such weak swimmers they should be thought of as
plankton. I think that the only way to keep one for any length of time
is in a specialized tank called a Kreisel, which is a circular tank
with a circumrotating current that prevents the animals from becoming
trapped against obstructions. Do Google searches on "Kreisel tank" and "Kriesel tank" (note misspelling) for more information.
There are a couple of other problems to think about:
1. Lifespan - Most jellies of a size appropriate for a home aquarium
are quite short lived. Don't be surprised if they die after only a
month or so. [The Moon jellies already mentioned have a total lifespan
of about a year in cold (10C) water, probably a good deal less in
warmer water. Plus, you really won't know how old they are when you
2. Food - Jellies of the appropriate type mainly eat zooplankton.
Copepods would be best but, since they don't live long anyway, you
might be able to get by with enriched brine shrimp.
Comb jellies (Ctenophores) are another possibility. They seem to be a
bit longer lived than jellyfish but are probably harder to feed since
they are smaller than jellyfish and have a more 'delicate' apparatus
for catching their food. We've kept these guys in our flow through
Kreisel quite successfully but I really have no idea what it is that
they eat exactly. Brine shrimp would be far too big and I even wonder
whether a ctenophore could subdue an adult copepod.
Jellyfish are very very brittle animals, it's even harder than trying to keep a ceph, in my opinon, I've kept a few jellies before, a upside down jelly lived for half a year before dying of a unknown cuase. But moon-jellies are hard! The best is to forget the powerheads and such and try to keep a single one in a salad bowl, that's how I got the best result...which was sadly still only two weeks.