Instructions:
Round tanks such as Biorb would work very well for the jellyfish. Fill the tank with ocean water and keep the temperature of the tank at roughly 21-29 oC . Keeping the water still won't allow the jellyfish to use diffusion, so keep water flowing to make current for diffusion process. For food, shrimps will work fine, and also shrimps can be bought easily in a supermarket. For breeding all you will need is a female jellyfish, if you have a male, then leave them in tank till their sperm and egg meet in the female internal cavity.
Reasons:
Jellyfish are amazing, they can breath through their skin, they can live forever, well for this jellyfish only of course. Talking about live forever, this is the only organism that can live forever as long as they wish to. If you had this as your pet you will never have to worry about them leaving you behind because they are immortal. Also, you can keep them as a family treasure, pass it down from one generation to another. Also as long as you have a male and a female you can have them breed and make more jellyfish for your family. Easy to reproduceand to take care, so that you have to put too much of your time to take care of it.