In ponds
Goldfish are popular pond fish, since they are small, inexpensive, colourful, and very hardy. In an outdoor pond or water garden, they may even survive if brief periods of ice form on the surface, as long as there is enough oxygen remaining in the water and the pond does not freeze solid.
Common goldfish, London and Bristol shubunkins, jikin, wakin, comet and sometimes fantail goldfish can be kept in a pond all year round in temperate and subtropical climates. Moor, veiltail, oranda and lionhead can be kept safely in outdoor ponds only in the summer, and in more tropical climates.
Small to large ponds are fine though the depth should be at least 80 cm (30 in) to avoid freezing. During winter, goldfish will become sluggish, stop eating, and often stay on the bottom of the pond. This is completely normal; they will become active again in the spring. A filter is important to clear waste and keep the pond clean. Plants are essential as they act as part of the filtration system, as well as a food source for the fish. Plants are further beneficial since they raise oxygen levels in the water.
Compatible fish include rudd, tench, orfe and koi, but the latter will require specialized care. Ramshorn snails are helpful by eating any algae that grows in the pond. It is of great importance to introduce fish that will consume excess goldfish eggs in the pond, such as orfe. Without some form of population control, goldfish ponds can easily become overstocked. Koi may also interbreed with the goldfish to produce a sterile hybrid fish.

In aquariums
The popular image of a goldfish in a fishbowl is an enduring one. However, some countries have banned the sale of bowls of that type under animal rights legislation due to the risk of stunting, deoxygenation and ammonia/nitrite poisoning in such a small environment. Because of their large oxygen needs and high waste output, the popular goldfish bowls of the past are no longer considered appropriate housing for goldfish.
Like most carp, goldfish produce a large amount of waste both in their feces and through their gills, releasing harmful chemicals into the water. This also happens because goldfish, like other cyprinids, lack a stomach and only have an intestinal tract, and thus cannot digest an excess of proteins, unlike most tropical fish. Build-up of this waste to toxic levels can occur in a relatively short period of time, which is often the cause of a goldfish's sudden death. Because of this, goldfish need a large volume of water in which to live. For common and comet varieties, each goldfish should have about 20 gallons(US) or 75 liters of water. Fancy goldfish (which are smaller) should have about 10 gallons(US) or 37.5 liters per goldfish. The amount of 'water surface area' is also important in determining how many goldfish may live in a container, as water surface area determines how much oxygen diffuses and dissolves from the air into the water. A general rule is have one square foot of water surface area for every inch of goldfish length (370 cm²/cm). If the water is being actively aerated by way of a water pump, filter or fountain, a smaller volume of water is needed.
The goldfish is classified as a coldwater fish, and can live in unheated aquariums at a room temperature comfortable for humans. However, goldfish cannot tolerate rapid changes in temperature (for example in an office building in winter when the heat is turned off at night). The sudden shift in temperature could kill them, especially if the goldfish are kept in a small tank. Care must also be taken when adding water, as the water added may be a different temperature than the water in the tank. Temperatures under about 10°C/50°F are dangerous to fancy varieties of goldfish, though commons and comets can be exposed to slightly lower temperatures without a problem. Extremely high temperatures (over 30°C/86°F) can also be harmful to goldfish. However, higher temperatures may be helpful for fighting protozoan infestations by accelerating the parasite's life-cycle — thus eliminating it more quickly. The optimum temperature for goldfish is around 25°C/77°F.
Like all fish that are kept as pets, goldfish do not like to be petted. In fact, touching a goldfish can be quite dangerous to its health, as it can cause the protective slime coat to be damaged or removed, which opens the fish’s skin up to infection from bacteria or parasites in the water. However, goldfish will respond to pet owners by coming to the surface of the water at feeding time, and can be trained or acclimated to taking pellets or flakes from a human's fingers. The supposed reputation of goldfish dying quickly is often due to poor care amongst uninformed buyers looking for a cheap pet. The true lifespan of a well-cared-for goldfish in captivity can extend beyond 10 years.
If left in the dark for a period of time, a goldfish will turn almost gray. Goldfish have pigment production in response to light, which is almost like our tanning in the sun. Fish have cells called chromatophores that produce pigments which reflects light, and gives coloration. The color of a goldfish is determined by which pigments are in the cells, how many pigments molecules there are, and whether the pigment is grouped inside the cell or is spaced throughout the cytoplasm. So if a goldfish is kept in the dark it will appear lighter in the morning, and over a long period of time will lose its color.

Feeding
Like most fish, goldfish are opportunistic feeders. When an excess of food is offered, they will produce more waste and feces, partly due to incomplete digestion of protein. Overfed fish can sometimes be recognized by feces trailing from their cloaca. Goldfish need only be fed as much food as they can consume in one to two minutes, and no more than three times a day. Contrary to some common belief, extreme overfeeding will not increase the size of the fish but can actually be fatal, typically by bursting of the intestines. This happens most often with selectively bred goldfish, which have a convoluted intestinal tract as opposed to a straight one in common goldfish. Novice fishkeepers who have newly purchased ryukin, fantail, oranda, lionhead or other fancy goldfish will need to watch their fish carefully for a few days, as it is important to know how much the goldfish will eat in a couple minutes of time.
Goldfish food has a lower protein and higher carbohydrate content than conventional fish food. It is sold in two consistencies - flakes that float at the top of the aquarium, and pellets that sink slowly to the bottom.
Goldfish enthusiasts will supplement this diet with shelled peas (with outer skins removed), blanched green leafy vegetables, and live food such as daphnia and bloodworms. Young goldfish also benefit from the addition of brine shrimp to their diet. As with all animals, individual goldfish will display varied food preferences. In any case, it is better to feed them a variety of foods listed above.

Memory
In 1994, a public experiment at the Palais de la Découverte science museum showed that goldfish could be trained to recognize and to react to light signals of different colors by using positive reinforcement. More recent research by the School of Psychology at the University of Plymouth in 2003 demonstrated that goldfish have a memory-span of at least three months and can distinguish between different shapes, colours and sounds. They were trained to push a lever to earn a food reward; when the lever was fixed to work only for an hour a day, the fish soon learned to activate it at the correct time. The Discovery Channel's popular show Mythbusters tested the contemporary legend that goldfish only had a memory span of 8 seconds and were able to prove that goldfish had a longer memory span than commonly believed. The experiment involved training the fish to navigate a maze. It was evident that they were able to remember the correct path of the maze and that they therefore had a memory span of over eight seconds. A study at the start of the 2000s showed that fish do actually respond to certain colours, this is most evident when it comes to feeding as fish are also aware of feeding times provided they are fed at around the same time everyday.