There’s nothing cuter than a goldfish—diminutive, bright and distinctly cheerful-looking, they’re a staple of fish tanks all around the world. But Australian scientists are not so enamored with the little darlings, reports Johnny Lieu for Mashable. Not only are they invading Australian rivers, but they’re growing to gargantuan sizes.

The huge goldfish of Western Australia are anything but adorable: Over the last 15 years, Lieu reports, they’ve taken to freshwater rivers in ever-greater number along with a host of other aquarium fish. In a new study published in the journal Ecology of Freshwater Fish, researchers reveal how the fish have spread throughout Australian waterways—and grown ever larger as they go.

The fish are not just big, the study found, they’re incredibly mobile. In just five days they can travel an average of one mile in the river. One intrepid fish went a whopping 3.35 miles in a mere 24 hours.

Over a year-long period, researchers tracked the movements of goldfish in the lower Vasse River, using acoustic testing and tagging to determine what fish were doing. The goldfish studied didn’t just swim around—they appear to have spawned in what ecologists call a “spawning migration,” a pattern in which fish breed in areas far away from their normal hangouts.

That’s bad news, Stephen Beatty, a senior research fellow at Murdoch University’s Centre for Fish & Fisheries Research who led the study, tells Smithsonian.com. “The fact that they’re so big is really symptomatic of the other impacts in the river,” says Beatty. The river, he explains, is warm and stagnant—perfect conditions for pet goldfish who make their way into waterways after being released by their owners. “The goldfish have really capitalized on that,” he says. Not only do the goldfish disturb the habitat and potentially consume invertebrates and fish eggs, his team suspects that they are also disease vectors.

Carassius auratus originated in Asia and are now kept as pets the world over. But when they’re released into the wild, the well-behaved fish tank friend becomes a foe to other wildlife. Not only do they grow without the constraints of a tank and commercial fish food, but their feeding frenzy causes mud and debris to rise from the bottom of the river. That in turn fuels the growth of aquatic plants, which can degrade the river even further. And while splashing around in the warm, nutrient-rich environment they love, they breed like crazy.

It's become an issue throughout the world: a Boulder, Colorado lake teems with the fish and in Alberta, Canada, the problem has become so bad that officials pleaded with the public not to release them. For Beatty, all that press is a good thing: “They’re a bit of a flagship because they do get that media attention,” he concedes. But their star status has a downside—a misconception that if your goldfish is tiny, it won’t hurt to drop it in a lake or river. “Introduced species can have really unpredictable impacts, even cute and fuzzy ones,” he says. “Please don’t release anything into rivers or wetlands that are not native there.”