1. Do you have enough money for a jellyfish tank? Jellyfish can be outlandishly expensive, even compared to other marine life. Jellyfish must live in circular tanks without any corners, to prevent damage to their tentacles, and their habitats require extensive filtration, a stable temperature, and meticulous chemical upkeep. "Desktop" jellyfish tanks, which are arguably far too small to accommodate any aquatic life, cost a minimum of $300-- and you will likely lose your jellyfish within just weeks in such a tiny tank. Sufficiently sized tanks cost anywhere from $3,000 to $20,000-- far more than most people are willing to spend on a high-maintenance pet with no central nervous system.
2. Do you have the experience necessary to keep pet jellyfish? Pet jellyfish are profoundly difficult to care for. If you're expecting the equivalent of a betta or goldfish, you're in for a serious challenge. Jellyfish tanks require careful chemical adjustment and a fairly advanced understanding of marine biology and water chemistry. Unless you have extensive experience keeping marine fish tanks, you are most likely unqualified to keep pet jellyfish.
3. Do you have enough time to invest into keeping pet jellyfish? If you think a dog or a cat is a lot of work, you've never had pet jellyfish. Jellyfish must be fed several times per day, and their water conditions must be checked on a daily or near-daily basis. Water changes for jellyfish are absolutely mandatory and can represent a significant time investment. While a dog-sitter or boarder may be easy to find, it can be very difficult to find a caregiver for your jellyfish when you go out of town.
4. Is it really worth it to you? At their core, pet jellyfish are not really pets. They have no brains, no personalities, and no capacity for companionship. Keeping a pet jellyfish can feel, fundamentally, much like keeping a very beautiful but extremely high-maintenance house plant. If the beauty of a jellyfish is worthwhile to you, a pet jellyfish can be a wonderful investment of time and money. However, if you want a real pet that can actually provide interaction and affection, you'd be better off with a guinea pig.
If you understand the difficulties involved in keeping pet jellyfish and still feel dedicated to the idea of keeping them, they may be an excellent option for you. However, jellyfish represent such a significant investment of time and money that that should not be purchased without significant forethought.