Rolf the pike versus abandoned goldfish: a predator has been released into a Gothenburg pond full of former aquarium fish, and why you should never set pets free

What happened

On 1 November 2026 the Swedish public broadcaster SVT showed a report on the Kvarndammen pond in east Gothenburg. It's home to large numbers of goldfish, former aquarium residents released by their owners. According to SVT, such abandoned fish cause serious problems in streams and ponds.

To reduce their numbers, a new "weapon" is being used in the pond: a pike named Rolf. The predatory fish is meant to eat the goldfish and so help the pond return to its natural state.

Why goldfish are a problem

The goldfish is a domesticated form of the Prussian carp, bred in Asia hundreds of years ago. It seems small and harmless in an aquarium, but in a pond with plenty of space and food it grows quickly and can reach the size of a large hand or more. Goldfish are undemanding, tolerate cold and poor water quality, and breed well.

In the wild they root around in the bottom for food and cloud the water, which worsens conditions for aquatic plants. They eat spawn, young fish, insect larvae and small water animals, and compete with native fish for food. Cloudy water rich in nutrients encourages algal blooms. As a result, a small pond can change beyond recognition in a few years.

Why a pike

The pike is a native predator in Swedish waters that ambushes other fish. The idea is simple: the predator reduces the number of goldfish naturally, without chemicals or draining the pond. This approach is sometimes used in small bodies of water, but it rarely gets rid of non-native fish completely. That usually takes a whole set of measures, including netting and sometimes draining the water.

How goldfish get into ponds

Owners themselves are usually to blame. The fish has grown, the aquarium has become too small, the children have lost interest or a house move is coming up, and releasing the fish into a pond seems kinder than giving it away or having it put to sleep. Some people genuinely believe the fish will be better off. In fact, it harms nature, and the fish itself often faces death from cold, disease or predators.

Can all the fish be caught?

Getting rid of goldfish in a pond is hard. They hide among weed and silt and breed quickly, and a few remaining fish can recolonise the pond in a couple of years. So specialists usually combine different methods and monitor for several years. It's far simpler and cheaper to keep goldfish out of the wild in the first place.

Not just goldfish

Other released pets cause similar problems: red-eared sliders, crayfish, guppies and other aquarium fish, as well as aquarium plants. Red-eared sliders, for example, live in urban ponds in many European countries, compete with native species and can carry salmonella. Non-native crayfish spread crayfish plague, which is fatal to native crayfish. In the UK, signal crayfish have devastated native white-clawed crayfish in this way.

Diseases and parasites

Aquarium fish released into the wild can bring bacteria, viruses and parasites to which native fish and amphibians aren't adapted. Sometimes these infections do more damage to waterways than the fish themselves. So conservationists urge people not to release any aquarium animals and not to pour aquarium water into natural waters.

What to do if you no longer want your fish

1. Offer the fish to friends or to local aquarium groups.
2. Ask at a pet shop: some accept fish or will help find a new home.
3. Find out about rescues for aquatic animals and turtles in your area.
4. Talk to a vet who treats fish if your fish is ill or old.
5. Never release it into a pond, river, lake or drain.

How to look after a goldfish properly

A goldfish doesn't belong in the small round bowl often seen in films. It needs a spacious aquarium with a good filter and aeration, regular water changes and checks on water quality. Goldfish eat a lot and foul the water heavily, so it's important not to overfeed them or overstock the aquarium. Many veiltails and other fancy varieties are prone to swim bladder disorders and need special care.

How to choose an aquarium for goldfish

• Volume: the bigger the better, as goldfish need plenty of water, especially adults.
• Filter and aeration: a powerful filter copes with large amounts of waste.
• Temperature: goldfish are coldwater fish and don't need tropical heat.
• Tankmates: don't keep them with tropical or small fish.
• Substrate and decor: with no sharp edges that could injure the fish.

What and how to feed

Feed goldfish a special food in small portions once or twice a day, as much as they'll eat in a couple of minutes. Leftovers foul the water. Bread and other table food isn't suitable. Cooked peas with the skin removed or other plant food can sometimes be good for them. Many fishkeepers give their fish a fasting day once a week.

Why water changes matter

Fish produce waste that turns into ammonia and other harmful substances in the water. A good filter with beneficial bacteria partly processes them, but nitrates still build up in the water over time. So part of the water is regularly replaced with fresh water that has been left to stand or treated with a conditioner. Aquarium water test kits help you check that everything is in order.

How long a goldfish lives

With good care goldfish live for more than ten years, and some even longer. It's worth bearing this in mind when buying a fish for a child: it isn't a temporary toy but a pet for years. If you're not sure you can look after a fish for a long time, it's better to choose other animals or not buy one at all.

A garden pond isn't a simple answer

Sometimes owners move grown goldfish to a garden pond. This can be a good solution if the pond is isolated and the fish can't reach natural waters, for example during a flood or via a stream. The pond must be deep enough not to freeze to the bottom in winter, and there shouldn't be too many fish in it.

What the law says

In many European countries, releasing non-native species into the wild is banned or restricted by nature conservation law and EU rules on invasive species. In Great Britain, the Wildlife and Countryside Act makes it an offence to release non-native animals into the wild. Even if a particular species isn't on a list, releasing an animal that could harm local nature is often treated as a breach. But the main point isn't fear of punishment: it's understanding that a natural waterway isn't a place for a pet.

How to spot a sick fish

Signs of illness in a goldfish include lethargy, refusing food, clamped fins, white spots on the body, a coating, ulcers, a swollen belly, loss of balance or swimming on its side. Poor water quality is often the cause. If your fish is ill, check the water, change part of it and contact a vet who works with fish or an experienced fishkeeper.

Before you buy a fish

Before buying a goldfish, consider whether you have room for a spacious aquarium, time for its care and the willingness to look after a fish for ten years or more. Work out who could feed the fish while you're on holiday. And decide straight away what you'll do if the fish grow or breed, so you won't have to choose between a pond and the death of your pet.

Children and aquariums

An aquarium can teach children responsibility, but care of the fish can't be left entirely to a child. Adults should explain that fish are living creatures with long lives and particular needs, and work out together who will feed them, change the water and clean the filter. They can also explain why a pet must never be released into a pond, however sorry you feel for it.

What this means for Europe

Abandoned goldfish and other aquarium animals are found in ponds and rivers across Europe, from Sweden to Spain. Local authorities spend time and money catching them, and nature pays the price. The story of Rolf the pike from Gothenburg is a good reason to remind owners that we're responsible for our pet fish for their whole lives, and that the kindest thing you can do for an unwanted fish is to find it a new aquarium.

When to see a vet

If your goldfish has stopped eating, stays near the surface or the bottom, has developed spots, a coating or ulcers, or is swimming on its side, check the water quality and contact a vet who treats fish. Many fish diseases can be cured if treatment starts early, but the wrong products from a pet shop can do harm.