Sliders were sold cheaply as hatchlings the size of a coin. Adults reach a shell length of twenty to thirty centimetres and can live for decades, which is more animal than most buyers planned on, and released turtles found many receiving waters warm enough to breed in.
Where they establish, they compete with native turtles for basking sites and food and can transmit parasites and pathogens. The IUCN Invasive Species Specialist Group includes them in its list of one hundred of the world's worst invasive alien species. The European Union has banned their import and sale, and other jurisdictions restrict them.
A persistent piece of misinformation is that the United States ban on selling small turtles was about invasiveness. It was a public health measure introduced in 1975 against Salmonella infection in children, and it applies to turtles under four inches regardless of species.