Giant Goldfish: Popular Housepet Can Grow Monstrously Large, As Tahoe Catch Shows

But hold on – can the tiny goldfish you win at a church carnival or buy for a dime at the pet store really grow up to become such a behemoth?

It turns out, yes!

The Guinness Book of World Records documented an 18.7-inch-long goldfish belonging to a Dutch man named Joris Gjisbers. Another gargantuan goldfish named Goldie made headlines in Britain. Goldie’s owner, an 83-year-old widow, said she bought the fish at a pet store when he was just an inch long. In 2008, Goldie was 15.7 inches long and weighed more than two pounds.

In January, fisherman Mike Martin pulled a 15-inch goldfish out of the waters of Lake St. Clair near Detroit.

The fancier name for the common domesticated goldfish is Carassius auratus. This fish is a member of the carp family and is native to China. That family resemblance between the carp and the goldfish means that sometimes people mistake the former for the latter – as seen when one French fisherman caught a 30-pound carp that some newspapers mistook for a goldfish.

While your goldfish is very unlikely to reach 30 pounds, it does seem that there’s some truth to the truism that goldfish can grow to fit their environment.

Wild goldfish are quite versatile, able to live in rivers, streams, ditches, lakes, and ponds and tolerate a broad range of temperatures. While most wild goldfish are a drab gray in color, red-gold colors do occur naturally, and have been propagated through breeding and housing techniques.

If you have the space for a big goldfish, your best bets for raising a giant one are to supply lots of high-protein food and warmth.

“Goldfish kept in outdoor ornamental ponds generally grow quickly in summer and little, if at all, over winter,” National Geographic scribe Carrie Arnold writes. “Keeping goldfish in a heated aquarium allows them to maintain their summer growth spurt year-round.”

But if you do decide on nurturing a giant goldfish, don’t dump it in the lake when it outgrows your tank. The Lake Tahoe discovery has scientists worried that giant goldfish will soon be outcompeting native fish and clouding the lake’s waters with their excrement, which promotes algal blooms.

While some threats to native lake species come from bait fish used by fisherman, the bulk of the problem comes from aquarium owners. One survey of fish-keepers in Texas found that between 20 and 69 percent would admit to dumping unwanted fish.

“Globally, the aquarium trade has contributed a third of the world's worst aquatic and invasive species," University of California, Davis, scientist Sue Williams told the site Our Amazing Planet in January.