MEDICAL LAKE, WA — Swim, fishy, swim, you may think as you free your little aquarium goldfish into natural waters. What's the harm? Plenty, it turns out. Wildlife officials are wrestling with a giant-sized goldfish problem at an eastern Washington lake, where the once-small fish have grown to the size of footballs and are choking out native species and stirring up all kinds of problems in the ecosystem.

The Washington Department of Fish and Wildlife estimates it may cost up to $150,000 to eradicate feral goldfish from West Medical Lake, and it's all because you thought you were doing a good thing for the fish, or simply no longer wanted to care for an aquarium. Goldfish are tiny members of the carp family, and are among the world's most invasive fish species. They have voracious appetites, compete with native fish for food and other resources, and cause a host of other problems, including encouraging algae blooms that disrupt the local ecosystem. They also introduce parasites and diseases that can threaten native species.

Supersized goldfish aren't just a problem in Washington, according to a Business Insider report that cited infestations in Europe, Canada and closer to home in Lake Tahoe, Nevada, where invasive species account for about half of the fish in the Lake Tahoe Basin. They reproduce quickly and with a yen for travel, can quickly migrate across bodies of water. For example, a few were released into an Australian river early in the 21st century and migrated to the Vasse River, where they've created a ginormous problem that continues today, according to one study that found fish traveled 3.35 miles in just 24 hours.

In Washington, wildlife officials are encouraging anglers to destroy goldfish if they catch them. "Goldfish are a common cold water aquarium fish that have been widely introduced throughout the U.S. via aquarium dumping," the Department of Fish and Wildlife says on its website. "After a few generations [they] revert to something very closely resembling the crucian carp. … As with carp, we do not want any new introductions, and would like those that are caught by fishermen to be destroyed."

Goldfish came to the United States by way of China, where they were selectively bred for food 2,000 years ago. They eventually went from the table to ornamental gardens, where they were regarded as symbols of luck and fortune, and came to the United States around the mid 19th Century.

Goldfish look fairly banal when swimming around in their small tanks, but they're actually quite smart and hold onto memories for about three months. Goldfish can discriminate between music by Bach and Stravinsky and can push tiny soccer balls into a net.