Pesky, oversized goldfish are causing problems in Minnesota.

Authorities in Burnsville, Minn., have urged residents and owners of pet goldfish, not to dispose of the family pet in lakes and ponds. The city tweeted a warning that doing so has resulted in the take over of one local lake by overgrown goldfish.

“They grow bigger than you think and contribute to poor water quality by mucking up the bottom sediments and uprooting plants,” authorities wrote on Twitter. “Groups of these large goldfish were recently found in Keller Lake.”

It’s not the first time Minnesota lakes became overrun with oversized goldfish.

Last November, wildlife officials found thousands of goldfish swimming in Big Woods Lake in Chaska, a suburb of Minneapolis. A team had to remove a truckload of 500,000 of the goldfish, due to environmental issues caused by the fish.

The problem has cropped up in Boulder, Colo., and Lake Tahoe, Nev., too, where researchers found thousands of goldfish in local lakes in both areas years ago.

The act of dumping unwanted goldfish into a local pond is actually illegal in most states, including in Minnesota where the problem has recently cropped up.

It’s considered “illegal fish stocking,” and its turned up in every corner of Minnesota. Doing so upsets the balance of existing natural, fish communities and spreads disease.

Goldfish are considered invasive species that uproot underwater plants and compete with native fish for food and shelter. Speedy reproducers, the fish live up to 25 years, and are a real pain to remove, according to Carver County, Minn., officials.

If you’re a pet owner and realized a decades-long commitment with your goldfish is not what you had in mind, the U.S. Fish and Wildlife Service recommends putting Goldie up for adoption. Another alternative is to contact a local veterinarian or pet retailer to find ways to humanely dispose of the fish, without causing harm to native fish species in your local neighborhood.