Goldfish: Interesting and Surprising Facts About a Popular Pet

Goldfish are common pets that have some surprising features. The colourful fish are found in many home aquariums, especially those designed for children. They’re often thought of as attractive but relatively simple creatures. Scientists and enthusiasts have made some interesting discoveries which show that there is more to the animals than is sometimes realized, however.

I had both indoor and outdoor goldfish as pets in my childhood and early teens. I enjoyed watching the fish and exploring their behaviour. Goldfish can make good pets. Unfortunately, they are sometimes kept under conditions that are far from ideal.

The goldfish has the scientific name Carassius auratus. It’s a member of the family Cyprinidae, which includes carp. The colour of the fish is believed to have first appeared—or at least to have first been noticed—in carp living in Ancient China.

The carp were raised in ornamental ponds or as food fish. A few of the normally silver fish had a mutation (gene alteration) that gave them a beautiful orange-yellow or golden colour. These fish were selectively bred to produce more coloured offspring as well as animals with fancy tails and other unusual features. The new varieties of carp were greatly admired. They eventually became known as goldfish.

Researchers at Plymouth University trained goldfish to press a lever to get food. Once the fish had learned to do this, the researchers changed the conditions of the experiment so that the lever released food only during a specific hour in each day. The goldfish recognized and remembered the time of day when pressing the lever was fruitful. They gathered around the lever during the correct time and pressed it to obtain food but ignored the lever at other times.

Jamie Hyneman from the MythBusters TV show also tested goldfish memory. He trained his fish to swim through an underwater maze consisting of multiple barriers. Each barrier had a hole surrounded by a red ring. The ring in a barrier was located at a different level from the one in the previous barrier. The goldfish remembered that they needed to travel through the holes to find food and reached the food faster over time. The experiment was continued for forty-five days.

Adam Savage from MythBusters also tested goldfish memory, but his fish weren’t successful in navigating the maze. Adam’s attempt shows how variables may affect the result of an experiment. He put different materials into the water from Jamie, which may have affected it chemically. In addition, he had a smelly waste problem in his tank and used green rings in the maze instead of red ones. All of the fish except for two died.

The surviving fish in Adam’s experiment may have failed the maze test for one or more reasons. They may have been harmed by the chemical or waste problem; they may have been unable to see green rings as well as red ones; and they may have had less chance to copy a successful fish’s behaviour, since only two animals survived.

Researchers at the Universities of Liverpool and Oslo have made an interesting discovery about a goldfish relative named the crucian carp (Carassius carassius). They’ve found that a major reason why the fish can survive under the frozen surface of a pond in winter is because of the alcohol that its body produces.

Like other fish and many other aquatic organisms, crucian carp and goldfish breathe by extracting oxygen from the water that flows through their gills. Under a layer of ice, the oxygen soon disappears. Most vertebrates die if they are covered by ice and have no way to escape.

Vertebrates such as fish and humans need oxygen so that their cells can produce energy via the appropriate chemical reactions. A small amount of energy can be produced without oxygen. Unfortunately, during this reaction lactic acid is produced. This substance quickly builds up to a dangerous concentration when no oxygen is available.

Members of the genus Carassius appear to solve this chemical problem by converting the lactic acid to alcohol, which enables them to survive. The researchers have discovered that their blood alcohol level becomes higher than the legal limit for driving in some countries. Apparently the alcohol is less damaging than the lactic acid because the fish can survive under the ice without oxygen for months. They remain conscious but aren’t very active. The scientists say that their discoveries apply to both the crucian carp and goldfish.

The existence of giant goldfish is not a myth. Assuming the animal eats enough food, the size of a goldfish is determined in large part by the size of its container, although genetics plays a role as well. If the fish are released into a large pond or a river, either deliberately or due to the flooding of an outdoor pond in a garden, they will likely become very big. That’s exactly what’s happening in some parts of North America and Europe, in Australia, and perhaps in other parts of the world. The fish are living and reproducing in the wild in areas as far north as the province of Alberta in Canada, which has cold winters.

The goldfish also lives and breeds in the wild in the United States. The USGS (United States Geological Survey) recognizes the animal as a non-indigenous aquatic species. The organization says that although orange individuals are occasionally found, the population of wild goldfish in the United States has mostly reverted to a pale olive-green colour.

The problem with releasing animals into an area where they don’t belong is that they may harm the ecosystem. A mature ecosystem normally develops some kind of balance that enables many species to survive. An introduced animal may alter this balance. For example, studies have found that introduced goldfish compete for food needed by other species. The fish are also expanding their diet. In one area they eat salamander eggs “with fervour”.

Goldfish are found in every pet store that I visit and are often inexpensive to buy. This seems to have led to some disrespect for the fish, which I find sad. I remember goldfish in plastic bags being given as prizes at fairs. Unfortunately, they still are in some places. Goldfish won’t live long if they’re treated casually and aren’t well cared for.

There are many things for a prospective goldfish owner to consider. These include:

  • tank size
  • a suitable location for the tank
  • gravel type
  • items to put in the tank
  • suitable food for the fish
  • a suitable water filter to remove waste materials
  • aeration (A water filter with a pump will probably provide enough aeration as it operates.)
  • water temperature
  • a method of water conditioning (Chlorine must be removed from water before fish come into contact with it.)
  • a method for cleaning the aquarium
  • a way to safely replace water lost due to evaporation
  • a method for gradually changing the water (Replacing the evaporated water is not the same as changing the water.)

Some people may be amazed to discover how big pet goldfish can get and how long they can live in a large tank and with proper care. In a small tank, goldfish may reach a maximum length of only around five inches and live for only a few years (or less). In a big tank, they may reach as much as ten to twelve inches in length —or even longer—and live for as long as twenty-five years. A few have lived into their thirties and early forties. These statistics are especially likely for the plainer varieties of goldfish, which seem to be hardier than the fancier kinds.

Several hundred different varieties of goldfish exist. They vary in colour, pattern, body shape, and features. Some varieties no longer look like goldfish. Some look like they have been inflated and/or have protuberances from various parts of their body. These protuberances may interfere with vision, swimming, or even breathing.

Selective breeding of domesticated animals can produce interesting and useful results. Some of the unusual characteristics of the fancy goldfish varieties are attractive and don’t interfere with the lives of the fish. I think it’s very wrong if breeding is done with the knowledge that it will probably make life difficult for the offspring, though.

In my opinion, fish shouldn’t be bred simply to satisfy people’s desire to have bizarre animals in their collection, especially when there is no concern about an animal’s quality of life. It would be far better for people to buy plainer and less expensive varieties of goldfish—which can be beautiful—and spend any money that’s left over on a large tank and items needed to keep the fish healthy and reasonably happy.

“Background Information About Goldfish” from the Bristol Aquarists’ Society

Four secrets your goldfish is hiding from you from BBC Earth

“Do fish really have a 3 second memory?” from The Naked Scientists (a group based at the University of Cambridge)

Another article entitled “Do Fish Really Have a Three-Second Memory?” from Mental Floss

Some members of the genus Carassius go months without oxygen by making alcohol inside cells from New Scientist

Extreme hypoxia tolerance in crucian carp and goldfish from Nature Scientific Reports

Facts about Carassius auratus from the United States Geological Survey