Behavior
Behavior can vary widely both because goldfish are housed in a variety of environments, and because their behavior can be conditioned by their owners.
Scientific studies done on the matter have shown that goldfish have strong associative learning abilities, as well as social learning skills. In addition, their strong visual acuity allows them to distinguish between specific humans. It is quite possible that owners will notice the fish react favorably to them (swimming to the front of the glass, swimming rapidly around the tank, and going to the surface mouthing for food) while hiding when other people approach the tank. Over time, goldfish should learn to associate their owners and other humans with food, often “begging” for food whenever their owners approach. Auditory responses from a blind goldfish proved that it recognized one particular family member and a friend by voice, or vibration of sound. This behavior was remarkable because it showed that the fish recognized the vocal vibration or sound of two people specifically out of seven in the house. Goldfish are gregarious fish that show schooling behavior, as well as displaying the same types of feeding behaviors. Goldfish may display similar behaviors when responding to their reflections in a mirror.
Goldfish that have constant visual contact with humans also seem to stop associating them as a threat. After being kept in a tank for several weeks, sometimes months, it becomes possible to feed a goldfish by hand without it reacting in a frightened manner. Some goldfish have been trained to swim through mazes, push a ball through a hoop, or even swim in a synchronized routine by their owners.
Goldfish have behaviors, both as groups and as individuals, that stem from native carp behavior. They are a generalist species with varied feeding, breeding, and predator avoidance behaviors that contribute to their success in the environment. As fish they can be described as “friendly” towards each other, very rarely will a goldfish harm another goldfish, nor do the males harm the females during breeding. The only real threat that goldfish present to each other is in food competition. Commons, comets, and other faster varieties can easily eat all the food during a feeding before fancy varieties can reach it. This can be a problem that leads to stunted growth or possible starvation of fancier varieties when they are kept in a pond with their single-tailed brethren. As a result, when mixing breeds in an aquarium environment, care should be taken to combine only breeds with similar body type and swim characteristics.

Native environment
Goldfish live natively in ponds, and other still or slow moving bodies of water in depths up to 20 m (65 ft). Their native climate is subtropical to tropical and they live in freshwater with a pH of 6.0–8.0,preferably a pH of 7.5, a water hardness of 5.0–19.0 dGH, and a temperature range of 40 to 90 °F (4 to 32 °C) although they will not survive long at the higher temperatures. They are considered ill-suited even to live in a heated tropical fish tank, as they are used to the greater amount of oxygen in unheated tanks. However, for centuries goldfish have been observed living in outdoor ponds in which the temperature often spikes above 86 °F (30 °C). When found in nature, the goldfish are actually an olive green color.
Fancy goldfish are unlikely to survive for long in the wild as they are handicapped by their bright fin colors; however it is not beyond the bounds of possibility that such a fish, especially the more hardy varieties such as the Shubunkin, could survive long enough to breed with its wild cousins. Common and comet goldfish can survive, and even thrive, in any climate in which a pond for them can be created. Introduction of wild goldfish can sometimes cause problems for native species. Within three breeding generations, the vast majority of the goldfish spawn will have reverted to their natural olive color. Since they are carp, goldfish are also capable of breeding with certain other species of carp and creating hybrid species.
Sudden changes in water temperature can be fatal to any fish, including the goldfish. When transferring a store-bought goldfish to a pond or a tank, the temperature in the storage container should be equalized by leaving it in the destination container for at least 30 minutes before releasing the goldfish. In addition, some temperature changes might simply be too great for even the hardy goldfish to adjust to. For example, buying a goldfish in a store, where the water might be 70 °F (approximately 21 °C), and hoping to release it into your garden pond at 40 °F (4 °C) will probably result in the death of the goldfish, even if you use the slow immersion method just described. A goldfish will need a lot more time, perhaps days or weeks, to adjust to such a different temperature.
In the wild, the diet consists of crustaceans, insects, and various plants. Because goldfish like to eat live plants, the presence of goldfish in a planted aquarium can be quite a problem. Only a few aquarium plant species can survive in a tank with goldfish, for example the Cryptocoryne and Anubias species, but they require special attention to ensure they are not uprooted. Artificial plants made of plastic are often more durable, but might irritate or harm a fish's skin if it comes in contact with the plants. Artificial plants made of silk are a reasonable alternative.

Breeding
Goldfish lay eggs. They produce adhesive eggs that attach to aquatic vegetation. The eggs hatch within 48 to 72 hours, releasing fry large enough to be described as appearing like “an eyelash with two eyeballs”. Within a week or so, the fry begin to look more like a goldfish in shape, although it can take as much as a year before they develop a mature goldfish color; until then they are a metallic brown like their wild ancestors. In their first weeks of existence, the fry grow remarkably fast - an adaptation born of the high risk of getting devoured by the adult goldfish (or other fish and insects) in their environment.
Some scientists believe goldfish only grow to sexual maturity if given enough water and the right nutrition. However, if kept well, they may breed indoors, but not in a small fishbowl. Breeding usually happens after a significant change in temperature, often in spring. Eggs should then be separated into another tank, as the parents will likely eat any of their young that they happen upon. Dense plants such as Cabomba or Elodea or a spawning mop are used to catch the eggs.
Most goldfish can and will breed if left to themselves, particularly in pond settings. Males chase the females around, bumping and nudging them in order to prompt the females to release their eggs, which the males then fertilize. Due to the strange shapes of some extreme modern bred goldfish, certain types can no longer breed among themselves. In these cases, a method of artificial breeding is used called hand stripping. This method keeps the breed going, but can be dangerous and harmful to the fish if not done correctly.
Like other common aquarium fish, such as the guppy, goldfish and other carp are sometimes added to stagnant bodies of water in order to reduce the mosquito populations in some parts of the world, especially to prevent the spread of West Nile Virus, which relies on mosquitoes to migrate. However, the introduction of goldfish has often had negative consequences for local ecosystems. As a result, goldfish are considered a pest in many countries, including the USA.

Controversy
Although edible, goldfish are rarely eaten except in certain countries. A fad among North American college students for many years was swallowing goldfish as a stunt and as an initiation process for fraternities. The first recorded instance was in 1939 at Harvard University. The practice gradually fell out of popularity over the course of several decades and is now rare.
In the Belgian city of Geraardsbergen an annual festival is held where the medieval tradition of swallowing a small fish swimming in a cup of wine still survives. There have been a couple of court cases in the last decade but the tradition seems to survive nonetheless. The fish used is often a goldfish because of its size and price.
In many countries, the operators of carnivals and fairs commonly give goldfish away in plastic bags as prizes for winning games. In the United Kingdom, the government proposed banning this practice as part of its Animal Welfare Bill, though this has since been amended to only prevent goldfish being given as prizes to unaccompanied minors. Rome, Italy, passed a law in late 2005, which banned the use of goldfish or other animals as carnival prizes. Rome has also banned the keeping of goldfish in goldfish bowls, on the premise that it is cruel for a fish to live in such a small space.
In Japan, during summer festivals and religious holidays a traditional game called goldfish scooping is played, in which a player scoops goldfish from a basin with a special scooper. Sometimes bouncy balls are substituted for goldfish.
In the UK, the Animal Welfare Act 2006 prohibits deliberate and “unnecessary suffering” to animals, but contrary to widespread belief, it does not explicitly outlaw the feeding of live feeder fish, such as goldfish, to other fish. However, it does prohibit introducing two animals for the purpose of “fighting, wrestling or baiting”.