Despite being in reasonably high demand, many owners are disappointed to find out that jellyfish don’t make good pets. Although they are pleasing to look at, jellyfish have several drawbacks that all interested aquarists should be aware of before setting up a jelly tank. Jellyfish can be expensive to keep, which is compounded by the fact that they are also delicate and short-lived.
1. They Have a Short Lifespan
The most common type of pet jellyfish is the moon jellyfish. This is because moon jellyfish are more readily available captive-bred, are easier to keep in a home aquarium, and are cheaper than their counterparts. However, pet moon jellyfish typically only live up to 12 months. While some species of jellyfish live a long time (like the immortal jellyfish), the varieties available for private aquarists are short-lived.
For owners who become deeply attached to their pets, such a short lifespan can be distressing. Often, the time invested in setting up a jellyfish tank is barely surpassed by the time the jellyfish actually live in the tank. Aquarists who are determined to keep a jellyfish tank will be faced with the unfortunate necessity of having to buy jellyfish on an annual basis, more than likely.
2. Jellyfish Are a Steeper (and More Frequent) Investment
As with most aquarium inhabitants, pet jellyfish have very specific care requirements, including certain tanks and diets. A moon jelly’s short lifespan, therefore, makes it a steeper investment than many other types of aquarium inhabitants. Whereas some fish or crustaceans may live for many years, moon jellyfish are considered old if they live past 12 months. Even aquariums that maintain ideal conditions often don’t prolong this short lifespan.
As a result, aquarists will have to continually spend money on not just food or cleaning supplies, but also the jellyfish themselves. While the associated cost isn’t steep— these jellyfish will likely be less than $50 apiece— it can still be frustrating when faced with fish that provide a better return on investment, as it were. This is on top of the costs associated with first setting up your home aquarium, since Kreisel tanks can be expensive.
3. Their Tanks Have to be Empty
Pet jellyfish have to be kept in a specific aquarium, called a Kreisel tank. Kreisel means ‘roundabout’ in German (or ‘spinning’ or ‘merry-go-round,’ depending on your translation). This is a reference to how the tank operates. Most aquariums simply hold water and rely on equipment, such as filters, to recycle the water. This means that the water typically flows in one direction.
But a Kreisel tank produces a very gentle, continually-rotating flow of water that mimics the rhythm of an ocean’s currents. The tradeoff is that these tanks have to be kept completely bare in order to preserve the water flow and keep the jellyfish safe in suspension. Even the tank’s lighting is meant to keep it bare. Moon jellyfish are kept under a specific type of blue lighting, called actinic lighting. While this does make jellyfish bioluminescence really pop, it also helps prevent algae growth.
4. Jellyfish Are Incredibly Delicate
Jellyfish are ephemeral, delicate pets that are mesmerizing to watch. But this beauty comes with a price, both literally and figuratively. Corners, sharp edges, and points can damage the jellyfish. Even something as simple as air bubbles can kill jellyfish if the bubbles become stuck under their umbrella. No plants, corals, or décor of any kind (not even aquarium equipment) can be allowed in the main part of the Kreisel tank with the jellyfish.
While the Kreisel tank itself is a feat of engineering, its intended purpose is somewhat boring: it keeps tank inhabitants in suspension. Originally designed for research purposes, the Kreisel tank has been adapted for home use and is now available online and in some specialty pet stores for private aquarists. The structure of the tank takes the delicate nature of the jellyfish into account: its rounded shape seems to discourage owners from adding anything except water, jellyfish, and food.
5. Captive Jellyfish Are Boring
Jellyfish are undeniably beautiful creatures, but they don’t tend to be very active or interesting in terms of behavior. Moon jellyfish primarily hang in suspension in the open water and their presence in a home aquarium reflects that. They simply allow the water flow in Kreisel tanks to carry them around and around… and around and around again.
Jellyfish cannot be trained like some other freshwater and saltwater inhabitants. And while some fish can recognize their owners, jellyfish don’t fall into that category. Additionally, they are a “hands-off” pet, and contact should be kept to a minimum. All of this means that jellyfish are a pet meant only for passive viewing, not any type of interaction.
6. These Pets Can’t Exhibit Natural Behaviors
Most aquariums are set up to not only allow but actively encourage natural behaviors. But for pet jellyfish, owners almost have to do the opposite. For example, while jellyfish can use their tentacles to trap prey in the wild, they are fed a diet of plankton, krill, and similar foods that can’t fight back in captivity. Exposing a jellyfish to prey that could hurt it might cause injuries, and aquatic vets are both hard to find and expensive.
Additionally, the Kreisel tanks themselves also discourage natural behaviors, in a way. Jellyfish can move horizontally and can pulsate their umbrellas, but they are less likely to exert this effort in captivity where the water flow keeps them in constant motion. Similarly, while jellyfish can move and hunt in groups, the typical home aquarium is not spacious enough to permit this.
