There's trouble lurking in the shallows of a neighbourhood stormwater pond in St. Albert.
Huge numbers of the pet-store paragons have infested Edgewater Pond, and have become a menace.
The fish have no place in Alberta waters, and conservation officials fear the invasive species will make its way into the Sturgeon River and harm the local ecosystem.
"The crazy thing about goldfish is they're so hardy, that's why they're so popular in the aquarium world," said Kate Wilson, an aquatic invasive species specialist with Alberta Environment and Parks.
"These species can actually pose a great risk to the environment, because they're so hard to kill."
Goldfish are so resilient they can crowd out native species, pillage local food sources, and introduce dangerous parasites, according to Wilson.
Even their poop is a concern, as it overloads the water with nutrients and encourages the growth of toxic algal blooms.
St. Albert has been working with Alberta Environment to find a solution since the orange invaders were detected in the pond last October.
The city has gone to extreme lengths to exterminate the problem population. Crews tried draining the pond so it would freeze to the bottom this winter. But mild weather failed to freeze them out.
Netting the fish was futile. Even electrocuting the water didn't work.
"The fish are doing great in that pond, it's a real concern," Wilson said during a Thursday morning interview on CBC Radio's Edmonton AM. "And goldfish multiply very quickly, much faster than our native fish."
Using poison on the pond has been discussed, but city officials fear the fish would only be reintroduced by unwitting pet owners.
"People who have aquariums, and don't want them anymore, instead of killing their fish or giving them away, they're dumping them in these ponds," said Wilson.
"People think this is a humane thing to do. But this practice is very harmful to native species, to Alberta waters, to our biodiversity and the fish that actually belong here."
City crews will monitor the situation this spring to determine the extent of the infestation and post signs at stormwater ponds that ask people not to dump fish in them.
They'll also look at putting a screen across the pond's outflow to try to hem in unwelcome water-dwellers.
Though the risk of the fish escaping the pond is considered low, the prospect of goldfish invading Alberta waters is a problem all municipalities should be prepared for.
"They can live in any water body in Alberta," Wilson said. "We've found them as far north as Fort McMurray, which is really cold. And these stormwater ponds are all connected to streams and rivers.
"So far we've only found them in our stormwater ponds, not our rivers or lakes. But it's just a matter time."