Common goldfish
The common goldfish is a type of goldfish with no other differences from their ancestors other than their color. Goldfish are a form of domesticated wild carp and are a close relative of koi. Most varieties of fancy goldfish were derived from this simple breed. Common goldfish come in a variety of colors including red, orange, yellow, white, black, and calico goldfish.
Common goldfish are social animals who prefer living in groups. They are able to interact with any fish belonging to the same species. With provision of adequate care and attention, common goldfish can become tame. Once familiar with the face of its owner, swimming towards the fish keeper during feeding time can be observed and hand-feeding becomes possible. Small goldfish will normally avoid any form of human contact. However, this fear ceases in a middle-sized and mature goldfish. A full-grown goldfish is more likely to eat directly from the hands of its owner without evident hesitation. While this behavior is welcomed by goldfish owners, it proved problematic in outdoor ponds where predators may eat such friendly prey. Mature goldfish will also explore their surroundings through nibbling or grazing behavior.
If transferred into a tank of other goldfish, a common goldfish would normally try to communicate and familiarize itself with its new tank mates by rubbing up against the body of other fish. The most common introductory gesture would be by swimming side by side with another goldfish with its head facing forward, or by swimming side by side with another goldfish with its head facing the opposite direction, or even by swimming above another goldfish in a perpendicular fashion. Schooling is a common behavior when there is a new fish in the tank. After some time, this schooling behavior eventually ceases, and soon every individual fish will again be swimming and exploring the aquarium on its own. Aggressive behavior is uncommon when a new specimen is introduced into a settled school of goldfish.
Hierarchy during feeding is commonly observed in which the larger goldfish receives most of the food. However, small goldfish may also become aggressive or competitive feeders despite the presence of larger fish which is, in general terms, considered a good sign, as a willingness to feed is indicative of a healthy goldfish.
It is a common practice to keep common goldfish in a small bowl but this allows waste in the water to build up to toxic levels and does not provide enough oxygen. For every small/young goldfish there should ideally be at least 10 to 20 US gallons (38 to 76 l; 8.3 to 16.7 imp gal) of water. A good filter, with no heater, is recommended because these fish can get larger than 12 inches (300 mm). Tank recommendations range up to 75 US gallons (280 l; 62 imp gal). It is possible to keep small goldfish in smaller tanks, but such an arrangement will be very difficult to maintain once the fish grows older. If there is concern about the fish not getting enough oxygen, particularly when it is warm, a water pump, such as a fountain pump, mini pond pump will pull the CO2 water from the bottom, expel it, and the surface action will oxygenate the tank or pond. Contrary to popular belief, air pumps and air stones do not oxygenate directly and rely on bubbles breaking the surface to transfer oxygen to the fish's environment. Ideally the water pump should push 10x the volume of the tank or pond plus an extra 100lph or gph. Goldfish will die without sufficient dissolved oxygen in the water. A filter that can do at least 10x filtration is best, which means that for every 10 gallons or litres of water, the filter should be able to cycle 100 gallons or litres per hour. If the oxygen in the water runs out then the fish may die or become unconscious. It is advised to move the fish to a basin of water full of fresh water.
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