Residents of the village of Edlington, near Doncaster, suggested the predatory fish could be to blame for a decline in ducks at Martinwells Lake, known locally as Brick Pond.
But experts have suggested that it is virtually impossible for the piranhas to have lived in Yorkshire-temperature open water.
A spokesman for Doncaster Council said the dead fish have been taken for examination.
"Given the natural habitat of piranhas, it is highly unlikely that the fish were alive at any time in the lake,” said Gill Gillies, assistant director of environment.
"We assume that these were pets that were placed in the lake, something that we would strongly discourage people from doing.
“The presence of a piranha is somewhat of a novelty so they have since been taken away by the Environment Agency for testing.”
An Environment Agency spokesman supported Ms Gillies’s statement.
"Piranhas are not native to England and do not belong in our rivers and lakes,” he said.
"These fish do not easily survive or feed at temperatures below 10C and readily succumb to the cold of northern winters.
"Nevertheless, we would encourage anyone that keeps exotic fish and which may have outgrown their tanks, to contact a specialist able to advise on how best to deal with them.
"It is illegal to release or transfer fish in England without the appropriate permit. It is also illegal to keep non-native species in the wild without a permit."
Despite both experts' doubts, Ms Gillies warned the public to err on the side of caution if planning a Bank Holiday dip.
"We would always strongly advise against anyone paddling or swimming in this or any lake due to the dangers of deep water," she said.
