A researcher from the U.S. Fish and Wildlife Service holds a giant goldfish found in Lake Erie.

U.S. Fish and Wildlife Service (giant goldfish); Mint Images - David Arky/Getty Images (fish bowl)

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Invaders

In a tank in your home, goldfish are harmless. But in the wild, they can grow to be gigantic problems.

September 29, 2025
By Cailin Loesch

As You Read, Think About: Why is it important to not release pet fish into rivers and lakes?

People couldn’t believe what they were seeing. In February, the U.S. Fish and Wildlife Service posted a photo of a goldfish on social media. The fish was nearly the size of a football! Researchers in Pennsylvania had found it while working in Lake Erie. 

Experts say what made for an attention-grabbing photo was also a sign of trouble. Goldfish in ponds, lakes, and rivers in the U.S. are an invasive species. That term refers to animals or plants that move into an area and harm the species that already live there.

Researchers estimate that tens of millions of goldfish could be living in U.S. waterways. They have been found in all 50 states—from warm ponds in Florida to chilly lakes in Minnesota. How did they get into so many bodies of water? And why do they grow so big?

People couldn’t believe their eyes. In February, the U.S. Fish and Wildlife Service posted a photo of a goldfish online. The fish was nearly the size of a football! Researchers in Pennsylvania had found it in Lake Erie. 

The fish made for an attention-grabbing photo. But experts say it was also a sign of trouble. Goldfish in ponds, lakes, and rivers in the U.S. are an invasive species. Invasive species are animals or plants that move into an area and harm the species that already live there.

Researchers say that millions of goldfish could be living in U.S. waterways. They have been found in all 50 states. They’re in warm Florida ponds as well as in chilly Minnesota lakes. How did they get into so many bodies of water? And why do they grow so big?

Wanted: Invasive Species
Watch a video to learn more about the damage invasive species can cause.

Gone Wild

Goldfish are a type of carp that’s native to Asia. They have been popular as pets in the U.S. since the late 1800s. But what happens when owners no longer want to care for their goldfish? 

Many people think the best solution is to release their pet into a pond or lake. But fish that have spent their whole lives in a bowl might not survive on their own.

“Many fish that have been bred in captivity aren’t suitable for living in the wild anymore,” says Matthew Neilson. He’s a biologist who studies invasive fish for the U.S. Geological Survey.

Goldfish are a type of carp. They are native to Asia. They have been popular pets in the U.S. since the late 1800s. But what happens when owners no longer want their goldfish? 

Many people think the answer is to release their pet into a pond or lake. But fish that have lived in a bowl might not survive on their own.

“Many fish that have been bred in captivity aren’t suitable for living in the wild anymore,” says Matthew Neilson. He’s a biologist. He studies invasive fish for the U.S. Geological Survey.

Food Fight

The fish that do survive can cause big problems. Goldfish reproduce quickly. Females can lay tens of thousands of eggs each year. And because goldfish have no natural predators in the U.S., it may not be long before they take over a body of water. 

“Populations of invasive species become harder to control as they become more abundant,” Neilson explains.

All these goldfish gobble up the food that native fish need to survive. As pets, goldfish eat a few fish flakes or pellets each day. But in the wild, they can become eating machines, devouring plants, insects, and even smaller fish. As a result, some goldfish grow to be more than a foot long and weigh more than 3 pounds! 

The way these supersized fish search for their next meal can also harm a habitat. Goldfish are bottom-feeders, meaning they search for food along the floor of a body of water. That stirs up sand and mud, making the water murky. Native fish and plants can struggle to survive in these conditions.

The fish that do survive can cause big problems. Goldfish reproduce quickly. Females can lay tens of thousands of eggs each year. And goldfish have no natural predators in the U.S. That means they can soon take over a body of water. 

“Populations of invasive species become harder to control as they become more abundant,” Neilson explains.

All these goldfish eat the food that native fish need to live. As pets, goldfish eat a few fish flakes each day. But in the wild, they can become eating machines. They devour plants, insects, and even smaller fish. As a result, some goldfish grow to be more than a foot long. They can weigh more than 3 pounds! 

The way these giant fish search for food can also harm a habitat. Goldfish are bottom-feeders. That means they look for food along the floor of a body of water. That stirs up sand and mud, making the water murky. These conditions make it hard for native fish and plants to survive.

Damage Control

Dumping goldfish into U.S. waterways is illegal. But there are already so many invasive goldfish in the wild that removing them all would be nearly impossible. 

Still, experts say pet owners can help prevent the problem from getting worse. That starts with never releasing goldfish into waterways in the first place. If you no longer want a fish, you can take it to a pet store or an aquarium. Or you might find a friend or family member who can give it a safe new home.

“When you put things in a body of water that aren’t supposed to be there, you could be hurting animals you love,” Neilson says. 

Dumping goldfish into U.S. waterways is illegal. But there are already many invasive goldfish in the wild. Removing them all would be nearly impossible. 

Still, experts say pet owners can help prevent the problem from getting worse. That starts with never releasing goldfish into the wild. If you no longer want a fish, take it to a pet store or an aquarium. Or find a friend who can give it a safe new home.

“When you put things in a body of water that aren’t supposed to be there, you could be hurting animals you love,” Neilson says.

Other Unwanted Guests

There are more than 6,500 invasive species in the U.S. Here’s a look at a few.

There are more than 6,500 invasive species in the U.S. Here’s a look at a few.

Burmese Python 

Shutterstock.com

The Invasion: The giant snakes were brought to the U.S. from Asia to be sold as pets. Scientists think many owners set the snakes free when they got too big to care for.

The Threat: Pythons have been slithering through South Florida since the 1980s. They’ve feasted on dozens of local mammal and bird species.

The Invasion: The giant snakes were brought to the U.S. from Asia to be sold as pets. Scientists think many owners set the snakes free when they got too big to care for.

The Threat: Pythons have been slithering through South Florida since the 1980s. They’ve feasted on dozens of local mammal and bird species.

Spotted Lanternfly 

Jason Ondreicka/Alamy Stock Photo

The Invasion: The colorful insects likely made their way from China aboard shipping containers. They were first seen in the U.S. in Pennsylvania in 2014. 

The Threat: Lanternflies harm a wide variety of trees and plants—including grape and apple crops. The insects suck the nutrients from plants and cover them with a sticky substance that can cause mold to grow.

The Invasion: The colorful insects likely made their way from China aboard shipping containers. They were first seen in the U.S. in Pennsylvania in 2014. 

The Threat: Lanternflies harm a wide variety of trees and plants—including grape and apple crops. The insects suck the nutrients from plants and cover them with a sticky substance that can cause mold to grow.

Wild Pigs 

Shutterstock.com 

The Invasion: European explorers brought the first pigs to North America in the 1500s. Over the centuries, countless pigs escaped from farms or were released into the wild to be hunted for sport.

The Threat: Millions of pigs are now running wild in the U.S. They trample crops and dig up dirt, destroying other animals’ habitats.

The Invasion: European explorers brought the first pigs to North America in the 1500s. Over the centuries, countless pigs escaped from farms or were released into the wild to be hunted for sport.

The Threat: Millions of pigs are now running wild in the U.S. They trample crops and dig up dirt, destroying other animals’ habitats.

1. According to the article, how did goldfish get into U.S. ponds, lakes, and rivers?

2. What does the article mean when it states that “the way these supersized fish search for their next meal can also harm a habitat”?

3. What is the purpose of the sidebar, “Other Unwanted Guests”?

1. According to the article, how did goldfish get into U.S. ponds, lakes, and rivers?

2. What does the article mean when it states that “the way these supersized fish search for their next meal can also harm a habitat”?

3. What is the purpose of the sidebar, “Other Unwanted Guests”?

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