When Good Pisces Go Bad: Don’t Release Your Tropical Fish!

This pacu, a fish from South America, was caught in a Midwestern stream. Photo by Author

Keeping an aquarium with tropical fish and plants can be fun and educational. But, releasing these fish or plants outdoors can be harmful to native species and is prohibited in Indiana and Illinois.

Many of the common tropical fishes and aquatic plants kept in home aquariums originate from Central and South America, Africa and southeast Asia. While some are imported, others are raised here, most notably in Florida. It doesn’t matter if they are made in America or imported – they don’t belong in Midwestern lakes and streams.

I’ve owned a home aquarium in the past and understand some kinds of these fish are expensive to buy and expensive to maintain. So why would people release them?

People may not be able to take their fish with them when they move, or they may simply lose interest in maintaining an aquarium. Some tropical fish also can grow too large for their aquarium. I suppose a few just accidently escape. Regardless, exotic fishes are released into the wild each year by their owners for many reasons.

There are many good reasons to not release aquarium fishes and plants into the wild. If they survive, and reproduce, they are difficult, if not impossible to control or eradicate. They can cause changes in the native aquatic environment by competing with native species. They can introduce exotic parasites and diseases. Some exotics may cause genetic damage by hybridizing with native species. And, some exotic fish such as piranhas, freshwater stingrays and electric eels can pose a human health threat.

Most tropical fishes are unlikely to become established in Midwestern waters due to our cold winter weather. Still, if they are out there, there’s a chance they could find a warm water pocket or migrate to where they could live. Many species are now breeding in the waters of southern states, and the goldfish, a native to China, is an example of a temperate aquarium fish that is now established in most U.S. states, including some Indiana and Illinois waters.

It’s not just fish (or reptiles – think pythons in the Everglades.) Aquarium plants such as Eurasian watermilfoil and variable watermilfoil have been found in many Midwestern states including Indiana and Illinois, with the Eurasian variety becoming dominant in several lakes. Hydrilla and Brazilian elodea are two other common aquarium plants that may be capable of surviving in around here.

Many tropical fish species have been found pretty close to home waters. Obviously these were recently released. For example, over the years biologists have found a South American oscars, a clown knifefish, native to southeast Asia and a pacu from the Amazon in local rivers.
Fortunately, being sub-tropical or tropical fish species, these fish cannot survive in temperatures below 55 degrees Fahrenheit so they all would have perished once winter arrived.

If one of these introductions had been a fish species capable of surviving the Midwest’s cold water temperatures, such as an exotic snakehead, it could have lived for years and created a real problem for the native species. They grow to large sizes and are voracious predators of other fish.

Releasing aquarium fish and many other fishes as well as aquatic plants is illegal and punishable by substantial fines. So what to do with them?

Flush them? It’s not a good idea. At best, a trip through the drains and a final dunk in a sewage plant or septic tank is invariably a final journey. In a city with combined storm and sewer drains, a fish would have a chance.

Just draining the tank and letting the fish flop around until they die would work, but seems somewhat callous. Better to just grab them and throw them hard on a concrete floor or driveway. They will be stunned until they die from oxygen depravation or killed outright. Another “humane” way is to put them in a freezer safe container and just shut the door on them. The water will slowly cool and they will die quickly in what I’m told is a swift and relatively painless process.

Aquatic plants should be cooked. Just letting them dry out might not work. Once they are boiled, they are dead and can be disposed of or put in the compost bin.

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