You should check the salinity of your jellyfish tank whenever you do a water change, which should be once a week.
Approximate completion time: 1 minute.
How to measure salinity with a hydrometer:
1. Fill the hydrometer with water from your tank.
2. Knock air bubbles from needle to verify salinity is between 30 and 34 ppt. Add fresh water or salt water as needed.
Note: Only use fresh water that has been distilled or has undergone reverse osmosis (as indicated on label).