Goldfish aren't the ho-hum fish you thought they were

Thank the Chinese for today’s beloved aquarium mainstay, the goldfish. A type of carp, goldfish were domesticated nearly 2,000 years ago for use as ornamental fish in ponds and tanks. They were seen as a symbol of luck and fortune, and they could only be owned by members of the Song Dynasty.

The fish are now ubiquitous in bowls throughout homes, classrooms, and doctor’s offices. They even share a name with a cracker, fondly known as the “snack that smiles back.”

Don’t confuse goldfish with its oversized cousin koi, another type of domesticated carp. There’s a common misconception that koi are large goldfish, but they are distinct species.

Not always gold

Goldfish weren’t always, well, gold.

Prussian carp, from which goldfish were domesticated, are traditionally a dull, gray-green hue. But mutations and breeding over the years created goldfish' signature orange, red, and yellow pigments found in the over a hundred varieties of the fish today. Goldfish first arrived in Europe in the 1600s and the United States in the 1800s, becoming what is likely the first foreign fish species introduced to North America. (Read more about the history of goldfish here.)

Goldfish have two sets of paired fins and three sets of single fins. They don’t have barbels, sensory organs some fish have that act like taste buds. Nor do they have scales on their heads. They also don’t have teeth and instead crush their food in their throats.

The fish are known for having large eyes and great senses of smell and hearing. Their ability to hear comes from small bones near their skull that link their swim bladder and their inner ear.

If you feel like a challenge, try counting the number of scales on a goldfish. It should have between 25 and 31. Then, estimate its length. According to the Guinness Book of World Records, the world’s longest pet goldfish is 18.7 inches and is owned by a man in the Netherlands.

In the wild

Goldfish go from cute to villainous when released into the wild. They’re known to carry disease and parasites, as well as breed with wild carp in the area.

Between 3,000 and 4,000 goldfish were discovered in a lake near Boulder, Colorado, in 2015, and researchers have even found large goldfish in Lake Tahoe.

The fish’s size is usually constrained by the size of its tank. But with enough food, proper water temperatures, and ample room to roam, goldfish can balloon.

People around the world occasionally pull in monster goldfish, a far cry from the miniature versions we’re accustomed to. A British teen caught a whopping five-pounder in 2010, and a fisherman on Michigan’s Lake St. Clair snagged a three-pounder in 2013.

Commercial fishermen on the Great Lakes have started making a profit from the invasive species. The almost 90,000 pounds of goldfish caught in Michigan in 2015 brought in nearly $70,000 in revenue.

In 2015, the Canadian government begged people to stop releasing their pets into ponds. As an invasive species, it can harm native fish populations by disrupting sediment with its feeding habits, scooting along the bottom of a body of water and stirring up dirt. Sometimes goldfish even eat the eggs of native critters, like salamanders in Idaho, or disturb vegetation other fish want to munch on too.

The U.S. Fish and Wildlife Service recommends pet owners who no longer want their goldfish either put it up for adoption (yes, furry friends aren’t the only pets that might get a second chance on life) or ask a local vet or pet store how to humanely euthanize and dispose of it.

There’s even an aquarium in Paris that takes in otherwise unwanted goldfish. Flushing them down the toilet? Mon Dieu!

An adaptable, intelligent fish

Goldfish are a hardy aquatic species. They can deal with temperature fluctuations, changes in pH, cloudy water, and even low dissolved oxygen levels.

If released into the wild, goldfish can group up into what’s called a school. But they don’t need companions to be happy in captivity and are fine if kept separately in a tank.

Because they’re not an aggressive species, they can be paired in a tank with fish that aren’t vastly different in size. They’re also smarter than may meet the eye. Researchers found that they can be trained to tell the difference between Bach and Stravinsky’s classical music.

In captivity, goldfish usually eat pellet or flake food. Supplements, however, are recommended to better mimic their natural diet. In nature, they eat worms, larvae, small crustaceans like brine shrimp, and even salad fixings like peas and lettuce. It’s recommended that goldfish owners add greenery to the bowl since the fish like eating live plants.