The carnivorous fish with rows of sharp teeth - which normally stalk the waters of the Amazon - were both discovered in the water in Doncaster.
Locals at the beauty spot say they have noticed a reduced number of ducks and fish at the lake - and think the flesh-eating fish could be responsible.
Residents found the carnivorous creatures at Martinwells Lake in Edlington, South Yorks and have taken photos and video.
Full-time mum Toni Hooper, 32, from Doncaster, saw the fish when out walking with her family on Sunday.
She said: "When we realised what it was it sent shivers down my spine.
"This is a popular spot amongst families, dog walkers and fishermen. It's always busy here.
"There's a play park nearby, so you get kids here paddling in the water, teenagers will go swimming here.
''You wouldn't catch me going in the water.
"We came here to feed the ducks and on Sunday we noticed there was only one duck and two ducklings, I'm concerned about where the wildlife is.
"I've spoken to others who have said they've noticed there aren't as many ducks."
Her partner, unemployed Gary Walker, 34, often fishes at the site and has noticed fish hauls have reduced.
The former clay pit is now home to a lake which is usually well-stocked with carp, tench, bream, perch, roach, pike and chubb along with usual pond life of newts, frogs and toads.
Ducks, coots and water hens all live on the pond which is visited each year by swans to raise their young. Even a group of geese had taken up residence there.
Another walker, mum-of-three Lisa Holmes, 37, who was there with partner, tree surgeon Davey White, 37, and their youngest child, Sonny, eight, were shocked to find the razor-toothed fish.
Carer Lisa, from Doncaster, said: “My partner is a fisherman and was looking around the edge of the lake when he suddenly spotted this fish floating near one of the pegs [fishing platform].
“He managed to get it out of the water and although he’s a keen angler, he wasn’t sure what type of fish it was straight away.
“But then we started looking at it more closely and saw the teeth we realised it was a piranha.
“We went home and Googled it and its quite clear its a piranha. It was quite a shock.
“We couldn’t believe that we’d found a piranha fish. It’s not the kind of thing you expect to find in Doncaster.
"We presume that it was a pet that someone no longer wanted and they have gone and dumped it in the lake."
More at home in the scorching climes of South America, these deadly man-eaters are certainly a fish out of water.
Helen Thompson, writing in the Smithsonian Magazine, said: "Piranhas attract a certain type of pet lover, and sometimes when the fish gets too large for its aquarium said pet lover decides its much better off in the local lake.
"In this manner, piranhas have shown up in waterways around the globe from Great Britain to China to Texas.
''Obviously it's never a good idea to release them into the wild, as the species could become invasive."
Known for their sharp teeth and powerful jaws, piranhas are known to have killed humans – although attacks on people are rare.
Attacks normally take place when the fish are stressed - such as when water levels are lower during the dry season and food is scarce.
Splashing can attract piranhas and for this reason, children are more often attacked than adults.
Most piranha attacks on humans only result in minor injuries, typically to the feet or hands, but they are occasionally more serious and can be fatal.
In 2011, a drunk 18-year-old man was attacked and killed in Bolivia, a five-year-old Brazilian girl was attacked and killed by a shoal in 2015 and in 2016 a Brazilian girl died after her grandmother’s boat capsized during a family holiday.
Various stories exist about piranhas, such as how they can lacerate a human body or cattle in seconds and when American President Theodore Roosevelt visited Brazil in 1913, he was shown the power of piranhas when he went on a hunting expedition through the Amazon Rainforest.
While standing on the bank of the Amazon River, he witnessed a spectacle created by local fishermen.
After blocking off part of the river and starving the piranhas for several days, they pushed a cow into the water, where it was quickly torn apart and skeletonized by a school of hungry piranhas.
Hit American horror B movie Piranha featured a deadly school of piranhas that escaped into open waters and killed unsuspecting swimmers.
The sightings have been reported to the Environment Agency.
Environment Agency have been approached for a comment.