That goldfish your neighbor “set free” in the lake behind the subdivision? Still out there. Possibly 18 inches long. Definitely causing problems.

New research published in the Journal of Animal Ecology confirms what ecologists have suspected for years: a pet goldfish released into the wild is a seriously bad idea. A team led by William D. Hintz of the University of Toledo built 32 miniature lake ecosystems, introduced goldfish, and documented the damage over 61 days. The results weren’t pretty.

In nutrient-rich water, the kind filling countless lakes across North America, goldfish drove floating sediment up by as much as 81 percent and cut water clarity by up to 65 percent. They do this by rooting aggressively through bottom sediment, consuming everything from algae to small animals to decaying plant matter as they go.

The invertebrates didn’t stand a chance. Snail populations fell 63 to 72 percent wherever goldfish were present. A shrimp-like bottom-dweller vanished at similar rates. Water fleas, tiny animals that anchor the aquatic food chain, dropped by as much as 92 percent when goldfish shared a tank with native golden shiners. Researchers think the two species hunt differently enough that together, they cleared out small drifting animals faster than either could alone.

Serious Damage Occurs in (Under?) Two Months

Native fish got squeezed out of their own food supply. Golden shiners living alongside goldfish came out about 12 percent worse off physically than shiners that never had to compete with them. Goldfish feed constantly and at exceptionally high rates for their size, which left the shiners with less of everything. For fish, that kind of physical toll compounds. A population 12 percent behind, year after year, eventually stops replacing itself.

Two months. That’s how long it took for measurable ecological damage to appear in a controlled tank. In a real lake, particularly a shallow or nutrient-dense one, the timeline could be even shorter.

Goldfish have lived in North American waterways since the mid-1800s, imported as pets, stocked in decorative ponds, and occasionally sold as algae controllers. Citizen-science sightings have climbed sharply over the last decade. Despite all of that, they get almost no attention in invasive species policy. Researchers are now urging wildlife managers worldwide to take goldfish seriously and pushing for plain-language warnings at the point of sale.

Releasing a pet goldfish feels like mercy. In the right lake, at the right nutrient level, it’s the start of a problem that outlives the sentiment.