At the Georgia Aquarium, Hall says that they mix 10 super-sacks at a time in 80,000-gallon mixing basins. They add it to Atlanta’s city tap water, which is first passed through activated carbon to eliminate the chlorine that keeps the water free of dangerous pathogens, but can be deadly to fish. At the bottom of the mixing basins, compressed air shoots out, agitating the mixture as if the water had come to a boil. The Georgia Aquarium mixes up an 80,000-gallon batch about every two weeks.

Working with such big volumes means the aquariums can’t take it for granted that everything will be mixed evenly. When the Shedd first switched over to making synthetic saltwater in the 1970s, it would sometimes find a big lump of undissolved salt in the tank. “We would have to send a diver in there to pull the lump apart,” a curator told the Chicago Tribune. In Chicago, water temperature makes a big difference, too. The city gets its drinking water from Lake Michigan, which can be close to freezing in the wintertime. “It could take a whole 24 hours for it to mix properly and to be nice and crystal clear. In the summertime, it might only take a matter of hours,” says Allen LaPointe, the Shedd’s vice president of environmental quality.

The National Aquarium in Baltimore sits right on the harbor, but it too makes synthetic saltwater for its marine mammals and fish. For one, the harbor water is brackish and not salty enough on its own. And even if the aquarium wanted to add salt to that water, the harbor is pretty polluted. Instead, the National Aquarium makes use of Baltimore tap water and a salt blend called Omega. The aquarium used to make its own salt blend from food-grade individual components. Making it in-house was cheaper, but more labor-intensive, according to Andy Aiken, the National Aquarium’s director of life support. “It also introduced the opportunity to make mistakes,” he says. For instance, forgetting the potassium (just 0.04 percent of seawater) could be disastrous for the fish that depend on it.

Aquariums that can pipe saltwater directly from the ocean do, and Monterey Bay Aquarium, in California, is the prime and enviable example. “Monterey is in this fantastic location,” Aiken gushed to me. “It’s everybody’s dream.” The bay is ringed by protected marine areas, so its water is exceptionally clean. At the back of the aquarium are two intake pipes that supply all the building’s saltwater tanks. “We’re literally physically connected to the bay,” says Kasie Regnier, the director of applied research at Monterey Bay Aquarium. The pipes can bring in almost 2,000 gallons of water a minute.

It works out great, except for the jellyfish problem. In certain years when the conditions are right, big swarms of jellyfish will appear in the bay. “They get sucked onto the screen. They’ll crush our screens,” says Regnier. The aquarium actually has to send divers down to unclog the intake pipes’ screens of jellyfish. (The divers, for their part, have gotten stung in the face.)