Never Release Aquarium Fish: Invasive Species and the Law

Why releasing aquarium animals and plants causes invasions, documented cases such as lionfish, snakeheads, plecos and Caulerpa, the main laws involved, and legal ways to rehome.

Why released aquarium animals become a problem

According to the USGS Nonindigenous Aquatic Species (NAS) program, most fishes sold in pet shops are not native to the country where they are sold, and each year more than 2,000 species, totalling nearly 150 million freshwater and marine fishes, are imported into the United States for the aquarium trade. The released fish are stressed, exposed to parasites, disease and native predators, and the release itself may be illegal.

If released fish survive and breed, USGS notes they are difficult or impossible to control or eradicate. They can compete with or prey on native species, carry exotic parasites and diseases, and hybridise with native fish. USGS reports that at least 185 exotic fish species have been caught in open US waters, 75 of them with established breeding populations, and that more than half of these introductions come from released or escaped aquarium fish.

Documented invasions

Lionfish in the western Atlantic

Two species, Pterois volitans and Pterois miles, have invaded the western Atlantic and Caribbean; about 93% of the invasive population is P. volitans. A lionfish was recorded off Dania Beach, Florida, as early as 1985. Because the captured fish resemble Philippine populations, release by aquarium hobbyists is the leading explanation. The fish spread north to Cape Hatteras and south through the Caribbean to Venezuela and Brazil by late 2014, with densities rising by up to 700% in some areas between 2004 and 2008. A separate Mediterranean population was first recorded in Israel in 1991. A single lionfish has been reported to reduce the number of juvenile reef fish on a reef by 79%.

Northern snakehead in the United States

In 2002 an angler found a northern snakehead (Channa argus) in a pond in Crofton, Maryland. A man admitted releasing two adults bought at a New York City market; officials drained the pond and destroyed two adults and more than 100 juveniles. All snakeheads of the genera Channa and Parachanna are listed as injurious wildlife under US federal rules (see below).

Sailfin armored catfish (plecos)

Several Pterygoplichthys species from South America are established outside their range: P. multiradiatus in Florida and Hawaii, P. disjunctivus in Florida, P. anisitsi in Texas, P. multiradiatus and P. pardalis in Mexico, and populations in the Philippines, Taiwan and Puerto Rico. Documented ecological damage is limited in some places; in Florida no measurable effect on native fishes has been found, although the fish can make up a large part of the fish community and their burrowing raises concern.

Caulerpa taxifolia, the aquarium strain

An aquarium strain of the green alga Caulerpa taxifolia was first reported in the Mediterranean in 1984 next to the Oceanographic Museum of Monaco; by 1999 eradication there was no longer possible. It was found near San Diego in 2000 and declared eradicated from Agua Hedionda Lagoon in 2006. The species appears on the IUCN Invasive Species Specialist Group list of 100 of the World's Worst Invasive Alien Species, which shows that aquarium plants and algae, not only fish, must never be dumped into natural waters.

Legal frameworks

United States: Lacey Act and 50 CFR 16

The Lacey Act, signed in 1900 and amended many times, has two parts relevant to aquarists. Its injurious wildlife provisions (18 U.S.C. 42) require a US Fish and Wildlife Service permit to import species listed as injurious in 50 CFR part 16, and to transport them between the continental US, Hawaii, Puerto Rico and US territories; a 1960 amendment extended these powers to fishes, reptiles, amphibians, molluscs and crustaceans. Its trafficking provisions (16 U.S.C. 3371–3378) prohibit importing, exporting, transporting, selling or buying wildlife or plants taken or traded in violation of US, state, tribal or foreign law, with civil and criminal penalties.

Section 16.13 of 50 CFR lists injurious fish and invertebrates, including walking catfish (family Clariidae), mitten crabs (Eriocheir), zebra mussels (Dreissena), snakeheads of the genera Channa and Parachanna, and several carp and other species. The same section states that imported live fish, molluscs and crustaceans, or their offspring and eggs, may not be released into the wild except by the state wildlife agency or with its prior written permission. Verify current status with USFWS and the state wildlife agency.

European Union: Regulation (EU) No 1143/2014

The EU Invasive Alien Species Regulation entered into force on 1 January 2015. Under Article 7, species on the list of invasive alien species of Union concern may not intentionally be brought into the EU, kept, bred, transported, placed on the market, used or exchanged, allowed to reproduce, or released into the environment. The first Union list of 37 species was published in 2016 and was expanded in 2017, 2019, 2022 and 2025; it currently contains 114 species. Article 31 allows non-commercial owners of companion animals acquired before a species was listed to keep them for the rest of their natural life, but only in contained holding where escape and reproduction are impossible. Verify the current Union list and national implementing rules.

United Kingdom

In Great Britain the retained version of Regulation 1143/2014 applies to invasive alien species of special concern, enforced through the Invasive Alien Species (Enforcement and Permitting) Order 2019, which came into force on 1 October 2019. Verify current status with the relevant national authority.

Alternatives to release

USGS NAS lists the following alternatives for unwanted aquarium fish, in order of preference:

1. Return the fish to a local pet shop for resale or trade.
2. Give it to another hobbyist, an office aquarium, a museum, a public aquarium or a zoological park.
3. Donate it to a public institution such as a school, nursing home or hospital.
4. If none of these is possible, have a veterinarian or fishery biologist euthanise it humanely with an anaesthetic.

Before giving away or selling a species, check that it is not listed as invasive or injurious where the recipient lives, since transfer or sale of listed species can itself be prohibited. Separate guides cover related rules on fluorescent transgenic fish, seahorse trade under CITES, seagrass protection and the US rule on small turtles.