They are seen as the ideal pet, mainly thanks to how little space they take up.

But outside of their small tanks and back in the wild, goldfish can grow to incredible sizes.

In fact, discarded goldfish are growing so big that authorities in the US are warning people not to release their unwanted pets into the waterways.

Football-sized aquatic invaders have been captured in Minnesota’s lakes after being dumped by their owners.

The City of Burnsville’s official twitter account shared pictures of goldfish that had been found in Keller Lake which required two hands to be held.

Officials said in July 2021: “Please don’t release your pet goldfish into ponds and lakes!

“They grow bigger than you think and contribute to poor water quality by mucking up the bottom sediments and uprooting plants.”

Officials in Burnsville — situated about 15 miles south of Minneapolis — found 10 fish last year, some measuring up to a foot long.

Local newspaper, The Star Tribune, reported that a second search resulted in another 18 being found, some of which reached 45cm and a staggering weight of four pounds.

Goldfish are related to the common carp and can grow to impressive sizes in the wild.

Kept in home aquariums, goldfish usually grow no larger than about 5cms in length — although there are exceptions.

But when released into the wild, what was once a tiny fish can balloon to an incredible scale.

Those swimming freely in open freshwater can often reach 30-35cm in length.

Hailing from east Asia, they can wreak havoc on lakes and rivers where they are not native, and even put local species at risk.

Many families have tales to tell about goldfish having short lifespans but, outside of captivity, they can prove to be remarkably hardy fish.

According to the website Live Science, they are capable of living for 25 years and can survive for up to five months without oxygen, thanks to evolving to live in ponds that freeze over in winter.

Matched with rapid reproductive rates, these eye-catching fish are able to quickly dominate new habitats — coming at the cost of native species.

The Washington Post reported that officials in Minnesota’s Carver County — 30 miles from Burnsville — removed about 50,000 goldfish from local waters in November 2020.

“A few goldfish might seem to some like a harmless addition to the local water body — but they’re not,” a statement from the Minnesota Department of Natural Resources said.

“Goldfish are in the minnow family and can work their way through city stormwater ponds and into lakes and streams downstream with big impacts, by rapidly reproducing, surviving harsh winters, and feeding in and stirring up the bottom like their close relatives, the common carp.”

Pet owners are being urged to give their unloved fish to pet stores or find someone else who can take care of them rather than throwing them into ponds.

“That’s really the last thing you should be doing,” Paul Moline, manager of the Carver County planning and water management department, told the Star Tribune.

Although pet goldfish are usually amber-hued or white with scarlet spots, most wild goldfish are a dull olive green colour that allows them to blend in at the bottom of ponds and lakes when hiding from predators.

According to the Guinness Book of World Records, the world’s longest domestic goldfish measured 47.4 cm from snout to tail-fin end.

Recorded in March 2003 in Hapert, The Netherlands, the pet was owned by Joris Gijsbers.

Last month, a British angler was celebrating after reeling in one of the world’s biggest open water goldfish at a fishery in France.

The gigantic orange specimen, nicknamed The Carrot, weighed a whopping 67lbs 4ozs — clocking in at nearly 5 stone (about the weight of an average 10-year-old child).

The fish caught by Andy Hackett, 42, was a hybrid species of a leather carp and a koi carp.