First off you would need a kreisel.
http://midwatersystems.com/productdetails.aspx?productID=3207
Fancy name for a special tank for jellies.
Secondly you would need a huge one. They are extremely expensive for small ones so a tank for a man of war would need to be insanely big.
Jellies are a bad choice of aquarium inhabitant. They are extremely fragile which is why the round tank is required. If the jelly were to bump in to a corner even very softly there is a good chance of it dieing. Then there is the hole getting stung part. Sometimes the the tentacles can break off and float in the water. Nematocysts which are the stinging cells can still fire even after the jelly is dead or the tentacle is detached from the organism. Jellyfish inject a extremely painful neurotoxin that has the potential to kill you. It destroys your nerves and without nerves your heart can't beat and you can't breath. Sound like a fun way to die? While most jellyfish (Schyphozoa, Cubozoa, and Hydrozoa) aren't deadly you always run the risk of an allergic reaction which would be deadly. So a jelly fish would be a bad type of organism to look after. Its best left to professionals and people with grant money to pay for a tank

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