UPDATE: Dead fish in Telford drains not piranhas after all, says Severn Trent
The dead fish - along with other breeds - were found lining pipes, causing a stink of a problem for homes in the Madeley area.
Today Severn Trent Water issued a warning to home owners that toilets should not be used to dispose of anything other than normal waste.
They believe the culprit was someone who became bored of their pet fish and decided to get rid of them by flushing them away.
The piranhas were dead by the time the workmen found them in sewers below the streets of Madeley.
It is the second time sewers have been blocked in Shropshire this year by rubbish being pushed into toilets.
Sewers overflowed in January in the town after several pairs of pants were flushed away in the Ditton Priors area.
Severn Trent said it was called to investigate the latest incident after people complained of a fishy smell coming into their homes.
Workmen Matt Final and Jay Slater, who went to investigate the smelly sewers for Severn Trent, said many of the fish were several inches long.
Mr Final said: "We've certainly seen some weird things in the sewers over the years but we were a little shocked to remove piranhas.
"You wouldn't think a fish of this size would fit down a toilet.
"It is just one example of amazing things we find blocking the sewers, but there is a serious aspect to this as they had the potential to cause a lot of damage."
Severn Trent today said blocked sewers are a serious issue. It said three-quarters of the 4,000 blockages in Shropshire last year were found to be caused by people misusing the sewer system.
The problem costs Severn Trent more than £10 million per year clearing products and mess from its network.
Sue Fulford, customer operations manager for Severn Trent, said: "Having sewage flooding inside your home due to sewers being blocked is one of the most horrible things that can happen to you, and we don't want our customers to suffer in this way, as it's entirely preventable.
"The drains that take waste water away from your home are only a few inches wide and are only meant to take water, toilet roll and human waste - certainly not piranhas or pants.
"Toilet roll is meant to break down when it gets wet so it washes easily through the system.
"That's not the case with things like sanitary products, cleansing wipes or, in this case, large tropical fish which can get stuck in smaller drains.
"These sewer blockages, if not identified and cleared, can lead to sewers backing up and overflowing into people's homes and gardens.
"And it's not necessarily the people who put the wrong things down the toilet that are affected.
"The blockage can happen further down the street, so what you do in your home can be causing a problem for your neighbours.
"We are spending millions of pounds every year sending teams into sewers with jetting equipment and sometimes even spades to dig out these blockages.
"We ask people to get themselves a bin in their bathroom and use that rather than the toilet."