Despite what some conspiracy theorists might shout at you, tap water is generally safe to drink for humans. For jellyfish, on the other hand, it might as well be poison.
Tap water contains small amounts of metals and minerals that you and I can hardly notice when we drink from the sink, but even in small concentrations, these elements can seriously disrupt a small jelly’s bodily functions and cause them to shrivel and stop belling. This is why we insist that whenever you add saltwater to your tank, whether you’re filling it for the first time or performing a routine water change, you ALWAYS mix the salt with either reverse osmosis (RO) water or distilled water, and NEVER use water from the tap, no matter how nice your filter pitcher might be.
Same goes for bringing your salinity down with freshwater: ONLY use RO or distilled when adding fresh water to your system.
*** Side note: bottled water is not always distilled water. Your jellies don’t drink Dasani! Jugs of distilled water are available at most grocery stores, but beware as there will be various types of water. Be sure it is labeled only as “Distilled Water” and not “Spring Water,” “Purified Water,” “Mineral Water,” “Alkaline Water,” or anything else. ***
Additionally, if your system maintenance requires filters to be rinsed periodically, it is best to do so with old saltwater taken from your system during a water change, rather than just running it under the sink. Your tank’s beneficial bacteria don’t care much for tap water either, and you certainly don’t want to make them unhappy.
The same rule applies with your algae scrubbers and any other items you might rinse and stick back in your tank. ALWAYS give them a quick dunk in some old salt water or some RO/distilled water and allow them to air dry before you place them back in your system. Your jellies will thank you!