Many were bought due to the craze for Teenage Mutant Ninja Turtles.
The tiny terrapins grow from 3cm to 30cm and can live up to the age of 50 aquariums can not take them under EU alien species laws
Dumping them in the wild is illegal. And aquariums are banned from taking more under EU alien species laws.
Owners have resorted to putting them down loos.
Alan Kwan, of Sea Life Manchester, said: “People don’t realise they grow from 3cm to 30cm and can live to 50.