Jellyfish are extremely delicate creatures and will not survie if it's not given the proper enviroment and live plankton. In addition to the tank and its filtration, jellyfish requires cold water. Since they live in waters that range from 55 to 65 degrees you will need a chiller. Pls read the links below or do a search with Google.
Can I keep Jellyfish at home ?
Rob Toonen - Jellyfish_
Plankton Kreisel Tank
Kreisel tank