D.C.'s National Aquarium Fills Tanks From City Tap

Most of the National Aquarium's tanks are home to saltwater creatures, so officials take the additional step of increasing the salinity of the treated tap water. They rely on a product named "Instant Ocean," a synthetic sea salt.

Aquariums near the shore have an easier time of it. The Monterey Bay Aquarium in California has a pipeline into the bay that provides its exhibits with seawater 24 hours a day at a rate of 2,000 gallons per minute, or 2.9 million gallons a day.

In some instances, said Monterey's husbandry curator, Paul Clarkson, officials put the seawater "straight on in" to exhibits such as its kelp forest, which features local vegetation. The water is nutrient-rich with larvae and invertebrates. "You'll see lots of sea life which we didn't put there," Clarkson said.

But visitors expect water that is clear enough for them to see the marine life, he added, so officials pump in filtered water in the daytime and unfiltered water at night at a rate that turns over the kelp forest's 330,000-gallon tank's volume least once every 90 minutes.

"If you look at the kelp forest exhibit during the day, it looks really clear and fabulous," Clarkson said. "If you come through at 2 in the morning, that raw water is coming in without any filtration, so it's a lot murkier."

Because the Monterey Aquarium returns its seawater to the Bay, it takes the additional step of ultraviolet sterilization to ensure it does not introduce invasive species into the bay. Without treatment, water from the exotic jellyfish exhibit, for example, could contaminate the bay and harm native species.

In a few instances, Monterey Bay officials use treated tap water for freshwater tanks, including a trout stream and an otter exhibit. They rely on the same carbon filtration system as the National Aquarium, along with ultraviolet sanitation, mechanical filters and a bacteria-containing sand filter to break down ammonia-based waste.

By the time this water goes down the drain, Clarkson said, "It's essentially pristine water going into the sewer system. We're accountable for every single drop of water that leaves the building."