Sure, goldfish look pretty adorable when they're at home in an aquarium, but they turn into an invasive, monster-size species in the wild. In 2013, researchers found a 4.2-pound, 1.5-foot long goldfish in Lake Tahoe. The massive fish wasn't alone either — there were enough smaller goldfish around that it was likely the fish were reproducing, according to Live Science. In Lake Tahoe, this resulted in two major issues: The massive goldfish were feeding on native species and excreting nutrients that increased the production of algal blooms, which in turn affected water clarity and cleanliness.
According to BBC, releasing goldfish is illegal in much of the U.S. because of this. Yet, that doesn't mean people aren't still doing it. In 2020, a small county in Minnesota removed 50,000 goldfish from a creek. Although it's hard to say how many were originally released and how fast they reproduced, the very large number present is a good indication of how invasive the species can be.
The problem is not just exclusive to the U.S., either. Europe is experiencing a similar issue in ponds and lakes. In 2010, a British teenager out fishing caught a 5-pound, 16-inch fish in a local lake in the U.K. (per BBC).
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