Fact-checking the Minnesota goldfish mystery: Scientists explain

The city of Burnsville has an urgent message for Minnesotans: Stop dumping your goldfish into the state’s waterways.

In a now-viral tweet posted on July 9, the city shared photos of goldfish: A resplendent orange, like a bag of nectarines, and the size of a football.

“Please don’t release your pet goldfish into ponds and lakes!” the tweet pleads. “They grow bigger than you think ...”

But how did these once tiny goldfish get so chunky, so large? The answer to that question lies in a complicated piece of advice passed down to fish owners, and a misunderstanding about pet goldfish: They should be able to grow to such a size.

Why should you never release pet goldfish?

Unable or unwilling to care for their pets any longer, goldfish owners in Minnesota have been releasing their fish into lakes and ponds in the state. The Burnsville city employees discovered after residents reported a potential goldfish invasion.

After being released into new waters, goldfish quickly become an invasive species that competes with native fish. They thrive in these Midwestern waters — perhaps a little too much, Brian Sidlauskas, associate professor and curator of Fishes at Oregon State University, tells Inverse.

“The species is native to Asia, not North America,” Sidlaukus says. Goldfish “can become quite invasive outside its native ecosystem, and competes with our native fishes for space and resources when released into North American waters.”

They will “keep growing until they die.”

Prosanta Chakrabarty, a curator of ichthyology — the study of fishes — at Louisana State University, tells Inverse goldfish can also harm native fish species through parasites and a voracious appetite. Goldfish don’t have specialized diets and as such, they can find plenty of food in their new waterways. This contributes to their enormous size.

“Goldfish, because they are generalist feeders, can really do a lot of damage to native species that are restricted to a specific diet, and the natives may not have previously encountered the parasites and diseases that the goldfish bring to an environment,” Chakrabarty says.

Goldfish can also pollute waterways by uprooting plants and sediment, according to the Burnsville tweet.

Burnsville isn’t the only Minnesota community suffering from a deluge of giant goldfish: The Wood Lake Nature Center in Richfield, Minnesota has attempted to reign in their invasive goldfish by releasing panfish and bass, which can eat goldfish. Invasive goldfish have also been spotted in Lake Tahoe, Nevada, and Boulder, Colorado.

How big will goldfish grow?

“They do get pretty big,” Chakrabarty says. Goldfish can grow to “more than a foot,” he reveals, and surpass three pounds in some rare cases.

Certain types of goldfish can grow to various sizes and shapes. Chakrabarty compares them to different dog breeds: “Like in dogs, there are different 'breeds' and there are the long-lived small breeds and the giant short-lived breeds.”

Culum Brown, a professor in the department of biological sciences at Macquarie University, tells Inverse a goldfish can reach up to 23 inches “if it's released into a large pond.” They can also live up to 30 years, though 10 to 20 years is more typical, Brown says.

That’s a long time to live in Minnesota’s waterways.

Do goldfish only grow to the size of their tank?

“Goldfish grow to the size of their tank” is positioned as a myth or sage wisdom, depending on where you’re looking on the internet. The theory is that these pets only grow as large as their home allows.

In some ways, however, this “myth” is true — though it’s more complicated than the idea you can give a goldfish a bowl however big you want based on their size.

“If you keep goldfish in small tanks, their growth is stunted,” Brown says. “If you transfer them to a bigger tank or put them in a pond, they grow very fast to catch up.”

They will “keep growing until they die,” Brown says.

In an aquarium setting, this idea becomes more complicated.

“Goldfish can grow to over one foot long, but when they are housed in small tanks with other goldfish, their growth is impaired by stress,” Lynne Sneddon, a senior lecturer in the department of biological sciences at the University of Gothenburg, tells Inverse.

She explains that “the small amount of space they have to move around in and the presence of others,” stunts their growth. As a result, pet goldfish may have poorer immune functions and reproductive systems, Sneddon says. Goldfish on the smaller side, in turn, still need a spacious tank.

In other words, the idea that a goldfish will grow to the size of its bowl can harm the goldfish — if you’re keeping a small fish in a small bowl. It’s not content with that space; it’s undergrown.

The goldfish seen in the Burnsville photos are actually “typical of fully grown members of their species,” Sidlaukus says. They grow in similar sizes as their close cousin, the Common Carp.

What should I do if I can’t take care of my fish?

While pet owners may think they’re being compassionate by releasing their fish into the wild, they’re actually harming their fish. (It’s also illegal to release your fish into the wild in most states).

“The worse thing is to introduce it to the wild where it can really do some harm or, more likely, die a slow painful death by freezing or malnutrition,” Chakrabarty says.

Instead, both Chakrabarty and Sidlauskas suggest giving your fish to another human that can care for it.

The US Fish and Wildlife Service provides a guide on how to safely “break up” with your goldfish, with tips on donating your fish to a pet store or school. If you cannot find anyone to care for your fish, you should consult with a veterinarian about how to humanly dispose of your fish.

If nothing else, Minnesota’s large goldfish offers a cautionary tale: Don’t ditch your pet fish in the local lake — for their sake and the survival of local wildlife.

“People should think twice about owning fish and understanding what their needs are,” Sneddon says.