In aquariums, jellies will grow according to the superiority of the animals. When keeping several jellies in a closed system, like an aquarium, the strongest jellies will emerge in the first two months and begin to get larger, while the others may grow more slowly or even start to get smaller. It doesn’t mean you are doing anything wrong in keeping them, it is simply a normal result of natural selection. Therefore, purchasing jellies that are significantly different in size makes for an uneven playing field when it comes to feeding. The larger animals you purchased will immediately outcompete the smaller ones, even though the smaller ones were originally just as strong.
Jellyfish in captivity, when maintained properly, will have a normal life expectancy of one year from start to finish. That being said, if you have a jelly that is 2” in diameter, it is already about 3 months old, so you will be able to enjoy it for about 9 more months. A jelly that is 3” in diameter is about 4 months old, and you will enjoy it for about 8 months, and so on. One year is one year—from the time of strobilation (the event where a jelly is “born”) to the time that it grows old and dies. So if you received it in January, let’s say, it is already 3 or 4 months old and will only live for another 8 or 9 months until September or October—not until the following January.
We always recommend initially purchasing jellies that are about the same size—within 1″ in diameter of each other. When jellies are more than 1″ apart in size, these competition problems can start to affect the smaller jellies.
If you start to notice a few of your jellies are shrinking, do not overfeed in an attempt to get them growing again. Feeding them more may seem like a solution, but it is not. The strongest animals will still win out over the weaker ones: the only thing you will be doing is compromising your water quality and the overall health of your setup. Please read more on our blog post about feeding and see some good examples of how much to feed. The best thing to do is simply let nature take its course and enjoy your jellies. Even though the strongest ones get bigger, the smaller ones will be just fine and simply exist “as is” in your system and give your tank some size diversity over time.