GEOFF BENNETT: For many people, it's their first pet, the humble goldfish.

Swimming in a tank in your house, it's hard to imagine it's much of a threat.

But when released into a great lake, a goldfish can imperil the fragile ecosystem of the largest freshwater system in the world.

Christopher Booker has the story.

ANDREA COURT, Wetland Restoration Ecologist, Royal Botanical Gardens: Yes, you can pull the bottom board.

CHRISTOPHER BOOKER: Ecologist Andrea Court is beginning another day at the Cootes Paradise Fishway in Hamilton, Ontario.

Sitting between Lake Ontario and the Cootes Paradise Marsh, this fishway acts as a border wall for fish.

The goal?

To keep invasive species out of this marsh, an important spawning and nursery area for native fish.

ANDREA COURT: We have a series of baskets or cages that are underwater.

So we have a crane.

It lifts the basket, dumps them into a container, and then we sort the fish.

At that point, I can decide which way the fish are going to go.

CHRISTOPHER BOOKER: With the flick of a wrist, Court sends native fish, like this channel catfish, into the marsh to spawn.

Invasives, like this common carp, aren't so lucky.

ANDREA COURT: We turn away anywhere from about 2,000, in recent years, to 9,000 carp.

CHRISTOPHER BOOKER: And when you say turn away, you send them back?

ANDREA COURT: Yes, I send them back to the harbor, yes.

They get denied entry into the marsh.

CHRISTOPHER BOOKER: When the fishway began operation in 1997, the main concern was invasive carp, which exploded in population in the marsh, displacing native fish and depleting aquatic plants.

But in the past decade, as carp populations have declined, a gilded icon of childhood pets has been making a splash.

ANDREA COURT: So, here we go.

Here's a goldfish.

It seemed like we moved the carp out and the goldfish kind of moved in.

So starting in 2013 was really when we started seeing more numbers, higher numbers of them.

CHRISTOPHER BOOKER: But these goldfish don't look like the friendly little guys swimming in tanks at your local pet shop.

Even their trademark color, bred into them in captivity, can disappear in the wild.

This is a goldfish, but it's not gold.

ANDREA COURT: Well, it's not very advantageous to be bright orange in the wild.

And so this is more of its natural color.

It can obviously be much more camouflaged.

CHRISTOPHER BOOKER: Kept in a bowl and fed a controlled diet, goldfish are small, unassuming pets.

But when they're released into lakes and ponds like this one, they gain access to an endless supply of food, and they can grow into large, destructive pests.

JON MIDWOOD, Research Scientist, Great Lakes Laboratory for Fisheries and Aquatic Sciences: They do grow quite large.

So I think the world record is nine pounds, you... CHRISTOPHER BOOKER: Nine pounds?

JON MIDWOOD: Nine pounds.

(LAUGHTER) JON MIDWOOD: But the biggest one that we have seen here is around 40 centimeters, so about half that size.

CHRISTOPHER BOOKER: Jon Midwood is a research scientist with The Great Lakes Laboratory for Fisheries and Aquatic Sciences.

He says these feral goldfish, which researchers estimate could number into the tens of millions in the Great Lakes alone, most likely originate from home fish tanks.

JON MIDWOOD: What we think is happening is that people have goldfish at home, have them in their aquarium, and when they're done, they don't quite know what to do.

And so they release them into the natural environment, because they don't want to kill that fish that they do love.

The same features or adaptations of goldfish that let them really thrive at home in that that bowl that sits on a shelf and doesn't get any new water for a while, isn't cleaned, those same adaptations let them thrive in these systems that are pretty degraded.

CHRISTOPHER BOOKER: He says goldfish are not only competing with native fish, but literally muddying the water around them.

JON MIDWOOD: What goldfish can do is, they will actually disturb the substrate or the sediment and mix it up, and it gets all murky and turbid, and that stops the vegetation from growing.

They're one of the most challenging fishes to have in the system.

There's other aquatic invasive species here, but we think goldfish are likely to have in the near future the biggest impact on that goal of trying to recover the aquatic vegetation.

CHRISTOPHER BOOKER: Midwood says he would like to see these invasive goldfish euthanized, but there's simply no safe and affordable way to do so, because of the contaminants they carry as bottom-feeders in an industrial waterway.

JON MIDWOOD: Those fish pick up a lot of those contaminants.

And so, if you're trying to remove them, there have been challenges in the past in terms of, where do you dispose of them?

Because they have heavy concentrations of things like PCBs and metals.

Once an aquatic invasive species is in your system, you can't do much other than try to keep its numbers down.

And so the best path is really prevention.

CHRISTOPHER BOOKER: But pet owners grappling with unwanted fish are often faced with a dilemma.

NEILEE IRWIN, Fish Owner: I had asked a few people that had ponds if they wanted them, but nobody wanted them, so I really didn't know what I was going to do with them.

CHRISTOPHER BOOKER: Across the border in Erie, Pennsylvania, Neilee Irwin was the reluctant owner of an unusually named goldfish that was quickly outgrowing its tank.

NEILEE IRWIN: He was named That One, because every time someone would come in the house, they would just go, "That one."

And I'm like, yes, that's his name.

He's the hugest.

CHRISTOPHER BOOKER: Under the right conditions, goldfish can actually live up to 30 to 40 years.

And this can leave their owners in deep water.

That's why the Erie Zoo started a one-of-a-kind pet project last year.

HEATHER GULA, Education Director, Erie Zoo: This is the Last Chance Lagoon.

This is the main pond.

CHRISTOPHER BOOKER: The Last Chance Lagoon gives unwanted goldfish and koi a luxurious new home, a 23,000-gallon tank in the heart of the zoo.

But according to Erie Zoo's Heather Gula, the primary function of the project is to educate the public.

HEATHER GULA: We want to teach them about invasives because that is a big problem on the Great Lakes.

We also want to teach them about proper pet care and expectations.

That is something that a lot of zoos work on educating the public about, because you have a lot of people that want exotic pets or bring in some of those that they just don't know how to care for.

CHRISTOPHER BOOKER: The Last Chance Lagoon intake forms tell the story of floundering fish owners.

HEATHER GULA: This ones name is Pumpkin Bob.

The owner did not realize how long they would live.

They did get Pumpkin Bob at a fair, and he was living in a one gallon tank.

CHRISTOPHER BOOKER: Along with about 50 other fish, Pumpkin Bob is now tank-mates with Irwin's fish That One, who's been living in the lagoon since last fall.

HEATHER GULA: It's That One.

Be free.

NEILEE IRWIN: They needed something better.

So I was very, very grateful, very grateful that I could put him in here, and now I can visit him.

CHRISTOPHER BOOKER: One less goldfish to wreak havoc in the wild.

For the "PBS NewsHour," I'm Christopher Booker at the Last Chance Lagoon in Erie, Pennsylvania.