“Please don’t release your pet goldfish into ponds and lakes!” The Twitter cry from the city of Burnsville, Minnesota, caught national attention this week, reported CNN.

  • Burnsville officials found almost 30 giant goldfish in Keller Lake over the last two weeks, CNN reported.
  • Everyone has been shocked by the size of these released household pets, per USA Today.

Goldfish are actually a very common invasive species throughout the U.S., and releasing goldfish into natural waterways is illegal in many states, reported USA Today.

How big can goldfish grow?

“They grow bigger than you think,” according to the city of Burnsville’s tweet. In a home aquarium, goldfish grow to be about two inches big, reported BBC.

  • But goldfish actually grow to match their environment, said USA Today.

So a small tank will have small goldfish, but a natural waterway ... that will have surprisingly large goldfish, as Burnsville officials (and much of the public) recently found out.

  • Burnsville officials found numerous goldfish the size of a football. The largest goldfish weighed four pounds and spanned 15 inches, per CNN.
  • The largest wild goldfish on record was caught in South Carolina last year and weighed nine pounds, reported USA Today.

Size aside, goldfish in the wild can actually cause serious problems, per BBC.

Why are giant goldfish an invasive species?

“All goldfish are invasive, basically worldwide,” said Przemek Bajer, a research assistant professor at the University of Minnesota and owner of Carp Solutions, per CNN. “In the whole of North America, Australia, parts of Europe, they are really, really widespread.”

  • Goldfish are native to China and East Asia, but their popularity as household pets has made them a common species around the world, reported CNN.

In the wild, goldfish can reproduce quickly and become a dominant species. They feed by digging around at the bottom of the lake, disrupting plant life and stirring up sediment into the water. Goldfish-infested lakes can go from having clear water to having green water, reported CNN.

  • Goldfish infestations typically start from residents releasing their household pets into the local waterways, said USA Today.
  • “You may think it’s a humane thing to do, but these fish are not meant to be in our natural system in large numbers,” said Daryl Jacobson, a Burnsville natural resources manager, per USA Today.

Currently, 49 states in the U.S. have reported goldfish invasions, per CNN. Alaska is the only state to have no invasive goldfish since they eradicated wild goldfish in 2019.

  • “We’re still in the process of trying to figure out what our situation is at Keller Lake,” said Burnsville natural resources specialist Caleb Ashling per CNN.
  • Burnsville is encouraging residents not to release their pet goldfish in local waterways. “Please consider other options for finding them a new home like asking a responsible friend or neighbor to care for it,” the city said on Facebook per USA Today.