The ubiquitous household goldfish can be found mindlessly swimming laps around fishbowls and aquariums worldwide, but when released into wild settings like Ontario lakes, these small fish can grow to absolutely gargantuan sizes comparable to an American football.

Fisheries and Oceans Canada has long warned pet fish owners not to flush or release their aquatic companions into the wild, sharing terrifying images of the resulting massive goldfish that have established themselves as a pesky invasive species in Ontario.

1/3 Think before you flush that unwanted critter down the drain! 🤔🐠

DYK that animals and plants intended for aquariums and artificial ponds can be harmful when released into the wild? pic.twitter.com/MIHyoJjZb9

— Fisheries and Oceans (@FishOceansCAN) March 3, 2023

In 2021, an absolute beast of a goldfish was pulled from Hamilton Harbour by a Fisheries and Oceans Canada research team, and similar finds have been noted across Ontario and British Columbia.

Ever wonder what happens to pet goldfish when they end up in our waterways? This one was pulled from Hamilton Harbour, where we’re studying this #InvasiveSpecies using acoustic transmitter tags. pic.twitter.com/GFCsBcIadV

— Fisheries and Oceans (@FishOceansCAN) November 30, 2021

But that isn't even the most terrifying part. The real horror is that there's a good chance that these enormous goldfish are not former pets released into the wild, but, instead, their descendants.

Fisheries and Oceans Canada tagged and tracked these colourful behemoths and discovered that they are reproducing using crucial spawning sites for native species, displacing existing fish populations and causing other ecological damage through their consumption of plant species and waste in the water.

By tracking these goldfish, we’ve learned that they’re breeding in Hamilton Harbour and targeting key spawning sites for native species like Northern Pike😱 - tearing up aquatic plants for food and clouding the waters with their waste. pic.twitter.com/KYb9JJrAfg

— Fisheries and Oceans (@FishOceansCAN) November 30, 2021

According to Brian Heise, an associate professor in the department of natural resource sciences at Thompson Rivers University, "the fish are hardy and reproduce quickly - releasing up to 50,000 eggs at a time, three times a summer."

released into ponds. That’s why it’s important to never 🚫release pets into the wild. Learn more about how to prevent the spread of aquatic #InvasiveSpecies https://t.co/QQXjySzQRn pic.twitter.com/9ucDQGR9ks

— Fisheries and Oceans (@FishOceansCAN) November 30, 2021

That's a whole lot of eggs, but it's not the only means of reproduction for feral goldfish.

Heise told the Canadian Press in March that "female goldfish have a special process called gynogenesis in which the female will get the sperm from a different kind of minnow… to start the eggs developing, even though they're not fertilized."

In layman's terms, they're cloning themselves.

As they spread, their potential for damage to the local ecology only multiplies. Fisheries and Oceans Canada says that released goldfish and their descendants "can carry diseases and infect or harm local species and the environment, even when dead."

"The solution is simple: Don't let it loose! Never release aquarium pets or plants into sewers or waterways, such as lakes, rivers, and the ocean."