The release of unwanted aquatic pets contributes significantly to the spread of invasive species, threatening native plants and wildlife, say researchers.

To tackle this issue, a three-step risk assessment method has been developed to identify high-risk species among freshwater pets, which can help restrict their release and prevent further invasions. Researchers from the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) in Berlin, Germany, have identified the most important risk species as the goldfish, guppy, and red-rimmed melania snail, due to their invasive nature and potential ecological impacts.

The spread of non-native species – animals, plants and other organisms introduced outside their native range – is one of the major drivers of biodiversity loss. Around a third of all invasive freshwater species originate from the pet trade, according to previous studies.

The researchers analysed the freshwater pet trade in Germany and identified 669 species, of which 651 - a whopping 97% - were non-native.

Fish were the most common group with 89.5% of all species, followed by molluscs with 4.3%, crustaceans with 3.9%, amphibians with 1.2% and reptiles with 1 per cent. Some of the species, such as the walking catfish, are on the list of '100 of the world’s worst invasive alien species'.

Although this study looked at Germany, this is an issue all around the world. The researchers recommend increased monitoring and public awareness to prevent the release of potentially impactful species into German waterbodies.

The legality of dumping a pet fish in a river depends on the specific laws of the jurisdiction in which you live. In general, however, it is not advisable to do so, as it can be harmful to the environment and the fish itself.

Pet fish are often bred in captivity and are not adapted to life in the wild. They may not be able to find food or shelter, and they may be susceptible to diseases that are not present in their home aquariums. Releasing pet fish into the wild can also disrupt the natural balance of the ecosystem and introduce harmful diseases to native fish populations.

Animal welfare organisation the RSPCA, which operates in England and Wales, says goldfish are not native to the UK and should not be released into waterways or ponds. Under the Wildlife and Countryside Act 1981, it is an offence to release or allow to escape any non-native species into the wild. Meanwhile, in the US, the Lacey Act prohibits the release of non-native fish into the wild.

If you are no longer able to care for your pet fish, there are humane ways to dispose of it. You can contact your local pet store or animal shelter to see if they can take the fish in, or you can find a responsible pet fish enthusiast who would be willing to take it over.