This is no crazy, urban, alligator-in-the-sewers kind of tale, giant goldfish are a very real thing and they're quickly taking over the water in Toronto and causing problems for everyone.
Most of these fish were once pets that lived in fishbowls or ponds and either escaped during flooding or were released into the public wetlands. And for anyone that ever thought goldfish couldn't be badass, these things are essentially the same as the dragons from Game of Thrones, and by that I mean they'll grow to whatever size their environment allows. Many of the ones that have been caught in the Toronto area have weighed around a kilogram.
Now, the problem with these massive goldfish is that once they find themselves in lakes and streams, they quickly uproot and eat the aquatic plants, ruining it for everyone else. They also multiply like you wouldn't believe and are crazy adaptable - basically unless they're trapped in solid ice over the winter, they'll survive. On top of that, they also have no natural predators, so these fish are living a pretty swell life.
Invasive species can become a big problem in any area, throwing off the delicate balance of an ecosystem. Alberta has had a very similar problem with giant goldfish, and has now imposed a $100,000 fine for releasing live fish into their lakes and rivers. If the situation in Toronto doesn't get under control, it might be a good idea to consider doing the same thing.