Authorities in Munich warned on Monday that setting free pet fish into public ponds and lakes is having a damaging effect on natural fish populations.
In the wild, the goldfish are forming aggressive shoals that are eating everything in the lake, leaving nothing for the indigenous population.
Because the native fish are quickly dying out, authorities have forbidden the release of any pet fish into the wild, threatening fines for anyone caught doing so.
The press release said that goldfish and other “ornamental fish” are rapidly overpopulating ponds and aquariums in people's gardens, eventually outgrowing their domestic habitat.
To ease the strain, owners are transferring their extra fish to public ponds instead of trying to take care of their overpopulation problems themselves, causing the shoals to form in the wild.
The press release said that it was the owners' responsibility to act, should their pet fish get too big for the pond. But authorities did give advice to any owners with fish overcrowding problems. They recommend that owners try and give away or sell their extra pets either over social media or through adverts in the local papers.
They also reminded owners that some pet shops also take any surplus animals back. For those looking for a more radical solution, authorities recommended that owners buy a predatory fish for their pond, which would stem the growth of the goldfish population.