Rehoming an animal you can no longer keep
Every keeper eventually has an animal they cannot keep: a fish that outgrew the tank, a species bought under the wrong name, a tank that has to be broken down. This is the practical guide to what to do about it, written because the release pathway is largely populated by people who wanted to do the right thing and did not know what it was.
Short answer
In order: a specialist keeper, a local aquarium society, a shop that will take it, a public aquarium, and — where nothing else is available and the animal cannot be kept adequately — humane euthanasia by a competent person. Release is not on the list, in any jurisdiction, for any species.
Start before you need to
The single most effective thing is not a rehoming route at all. It is knowing an animal's adult size before buying it, because most of the difficult cases are fish that were sold small and were always going to become impossible.
A common pleco at six centimetres reaches fifty. An iridescent shark at five reaches a hundred and thirty. A clown loach lives for decades and reaches thirty centimetres. A redtail catfish exceeds a metre and forty kilograms, and almost no private aquarium in the world can hold an adult. Those are not obscure species; they are stocked in general pet shops.
The routes, roughly in order
Not every route is available everywhere, and the order below is about likelihood of a good outcome rather than convenience.
- A specialist keeper. Local and regional aquarium societies exist in most countries and are the best route for anything unusual. A society member who wants a 40 cm oscar has a tank for it.
- An aquatic shop. Many will take fish back, some will credit them, and a specialist shop is far more likely to than a general pet chain. Ask before assuming.
- A public aquarium or zoo. Realistic only for genuinely large or unusual animals, and they receive many more offers than they can take. Ask rather than deliver.
- Online rehoming within the hobby. Works well for species with an enthusiast following and badly for the large catfish that most need it.
- Humane euthanasia, where nothing else is available and the animal cannot be kept adequately. This is a legitimate last option and it is a better outcome than years in a tank too small, or a release. Clove oil overdose is the method most commonly described for fish; a vet with fish experience is better where one is reachable.
What not to do, and why each one specifically
Do not release into a pond, river, lake, canal or the sea. This is the pathway that produced the walking catfish in Florida, the topmouth gudgeon in Europe and the red swamp crayfish on four continents. In many jurisdictions it is a criminal offence.
Do not flush. It does not spare the animal anything and it does not reliably kill it — several aquarium species survive sewage systems, and the ones that do not still carry pathogens and eggs into the outflow.
Do not tip tank water down an outside drain. Storm drains reach watercourses in most towns. Aquarium water carries snail eggs, plant fragments, larvae and pathogens whether or not it contains a fish. Waste water goes down a household drain into the sewage system, not into the street.
Do not compost aquarium plants outdoors or put them in green waste. Several of the most damaging aquatic invasives in Europe and North America are aquarium plants, and a fragment is enough.
Do not release into a garden pond that connects to anything, including in flood.
Snails, shrimp and plants are the same question
It is easy to think of this as a fish problem. It is not. The Malaysian trumpet snail's establishment worldwide is attributed partly to hobbyist releases and partly to arriving on plants. Apple snails are the subject of an EU plant-health restriction on the entire genus because of what they do to aquatic vegetation.
Invertebrates and plants are also the organisms most likely to leave a tank accidentally, in transferred water, on nets, or as fragments in a bucket. Treating them with the same care as a fish is the whole of the biosecurity advice.