Salt Water vs. Fresh Water Aquariums
Moon jellyfish require salt water, but it is only a few extra minutes per month to maintain the salinity.
There is a common rule of thumb among aquarium stores that salt water tanks are more difficult to maintain than fresh water tanks. However, this is not necessarily true. Jellyfish tanks are easier to maintain than many fresh water tanks because jellyfish have such a low metabolism.
Salt water tanks can be difficult to maintain only if you keep very advanced, exotic species. However, having a salt water tank does not mean you have to keep difficult fish. For example, the Red Legged Hermit Crab is a saltwater creature but is very hardy.
The only extra work required by every salt water tank is maintaining the salinity. Our jellyfish tanks have very low evaporation rates, so maintaining salinity takes a few minutes per month.
Simply check the salinity before doing a water change.
If it is near the maximum threshold for jellyfish (1.028), add fresh water in addition to the replacement salt water. This will bring the salinity down.
If salinity is near the minimum threshold for jellyfish (1.024), add some aquarium salt in addition to the replacement salt water. This will bring the salinity up.
Be sure not to use tap water in a jellyfish tank. You must use reverse-osmosis filtered or distilled water or water, which can be found at aquarium stores.
Salinity will only go up as fresh water evaporates, so once your salinity is within 1.024 and 1.028, you will never need to add aquarium salt. Once you have salinity above the minimum jellyfish threshold of 1.024, you don't have to worry about the salinity being too low again.
Don't let the misconceptions about salt water tanks keep you from trying jellyfish. Caring for salt water fish can range from very easy to very difficult and luckily jellyfish fall near the easy end of the scale.