Large goldfish, such as these pulled from a waterway near Niagara Falls, are evidence that individuals are releasing aquarium fish in the environment. (The Minnesota Star Tribune)

Giant goldfish are becoming a problem in Minnesota lakes, and wildlife officials are warning fish owners who no longer want to care for their pets not to flush them down the toilet or dispose of them in lakes, ponds or waterways.

Donate them instead.

"They can grow to be the size of dinner plates in these waters," said Sharon Moen, a spokesperson with the Minnesota Sea Grant.

Moen said wildlife officials have recovered giant goldfish, piranha, Amazonian catfish, koi and even a caiman — a reptile related to alligators — from Minnesota waterways. A pond in Duluth was taken over by goldfish, she said.

Goldfish have been recovered from Powderhorn Lake in Minneapolis; the Department of Natural Resources also caught one near Albert Lea that was 9 to 11 inches long.

"It's not that common," she said. "But it happens more than you would think."

Releasing fish and plants from aquariums can harm Minnesota waters and native species. Fish can carry diseases, reproduce quickly and root up native plants to find food.

It is illegal to release aquarium plants and fish in Minnesota, according to the Department of Natural Resources. Aquarium fish can carry diseases that can kill native fish. Invasive plants such as water hyacinth and yellow iris can clog waterways and snag boat propellers.

And flushing fish down the toilet will not end in a "Finding Nemo" moment.

"If you flush, they go to a wastewater treatment plant," Moen said. "There are kinder ways to euthanize the fish."

According to fishchannel.com, fish less than two inches long can be euthanized by exposing them to freezing cold water to render the fish unconscious. Clove oil also also be used.

The best solution for getting rid of unwanted living aquarium fish and plants is to donate them. Minnesota Sea Grant and the Minnesota Aquarium Society are hosting the first-ever Habitatattitude Aquarium Fisha and Plant Surrender Saturday at Lutheran Church of Redemption, 927 E. Old Shakopee Road, Bloomington.

Surrendered fish and aquarium plants can be dropped off from 10 a.m. to noon; any fish or plant will be accepted with no questions asked. The fish and plants will be auctioned from 11 a.m. to 6 p.m. The proceeds from the auction will support the Society's efforts to promote "excellence in fish keeping."

Aimee Blanchette • 612-673-1715