All the people heard about the "golden fish" and are able to recognize one in pet-shops, but there are lots of species, some of them are hideous, others very beautiful. Also, not all the species are compatible between them.
Temperature: 10°C - 24°C (50°F -75°F). If the water is under 15°C (59°F) they stop eating. More demanding species (oranda, veiltail, black moor, etc) need temperatures of 18°C – 26°C (64°F - 78°F), otherwise they will die.
pH: 7.2 – 7.6. On short periods of time a goldfish can resist at values between 6.0 and 8.0, but on long terms he needs a neutral pH.
Water hardness: doesn’t matter.
Compatibility with other fishes. You can’t keep a goldfish with other species of fish, not even with Ancistrus or Neon tetra (Paracheirodon innesi). Demanding goldfish species must be kept with other demanding species only, and not with common species.
Tank dimensions: 40 liters (10 gallons) per each. For 2 goldfish will be needed 80 (20 gallons) real liters, so the tank must have 110 liters (30 gallons).
Feeding. Goldfish specific food has less protein and more carbohydrate than conventional fish food. It is sold in two consistencies, flakes that float and pellets that sink. Enthusiasts may supplement this diet with shelled peas (with outer skins removed), blanched green leafy vegetables, and bloodworms. Young goldfish benefit from the addition of brine shrimp to their diet. Attention, overfeeding can be fatal, typically by blocking the intestines.
Life duration: 10-25 years.
Behavior. Goldfish have strong associative learning abilities, as well as social learning skills. In addition, their visual acuity allows them to distinguish between individual humans. Owners may notice that fish react favorably to them, while hiding when other people approach the tank. Goldfish that have constant visual contact with humans also stop considering them to be a threat. After being kept in a tank for several weeks, sometimes months, it becomes possible to feed a goldfish by hand without it shying away. Goldfish have learned 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. As fish they can be described as "friendly" towards each other. Very rarely does 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 competing for food.
Reproduction. Goldfish are egg-layers. To provide an ideal aquarium environment for the fish to spawn, it is best that the water should be crystal clear and clean, fresh from water changes and the tank bottom should be bare without sands or gravel. The water temperature must be maintained cooler than normal aquarium setup for goldfish which is at least 2 – 3 degree Celsius lower than what you normally control in your tank. Their eggs are adhesive and attach to aquatic vegetation, typically dense plants such as Cabomba or Elodea or a spawning mop. The eggs hatch within 48 to 72 hours. Within a week or so, the fry begins to assume its final shape.
Varieties of Goldfish:
Common Goldfish - elongate shape, simple tail, dorsal fin, variety of colors including red, orange/gold, white, black and yellow or “lemon” goldfish.
Comet Goldfish - elongate shape, long and deeply forked tail, dorsal fin, metallic colors, can grow till 30 cm.
Wakin Goldfish - elongate shape, double tail, double anal fin, dorsal fin, only metallic colours (orange, silver or combinations).
Veiltail Goldfish – extra long, flowing double tail, dorsal fin,divided double anal fin, metallic or mat color. A standard veiltail goldfish has the tail formed of two triangular lobes , but the most variaties have the two lobes bifurcated. Can grow 15-17 cm. Can’t be kept in lakes because they die at low temperature. They aren’t good swimmers because of their long tail and is not recommended to be in the same tank with Common Goldfish or Fantail Goldfish.
Oranda – rounded body, double tail, dorsal fin, double anal fin, metallic or mat color, headgrowth. Usually is orange but may also be white with headgrowth red, black. Most appreciated are those with two triangular lobes. It is not recommended water temperature to be below 18°C (64°F) because at 15°C (59°F) they die. They are less resistant to diseases, especially those caused by bacteria. Can grow 17-20 cm.
Black Moor – telescope eyes, long double tail, dorsal fin, double anal fin, only black color, two triangular tail lobes. Can grow till 20 – 25 cm, being one of the largest goldfish.
Fantail Goldfish – long quadruple caudal fin, high dorsal fin, double anal fin, egg-shaped body and no shoulder hump. Can grow 15 – 17 cm.
Pearlscale – spherical body with fin similar to the fantail.
Lionhead – without dorsal fin, headgrowth, short tail, double anal fin. They not grow more than 10 – 12 cm, being one of the smaller goldfish.
Bubble Eye – without dorsal fin, upward pointing eyes accompanied by two large fluid-filled sacs. Not to be confused with telescope eye.
Ryukin – short tail, deep body with a characteristic shoulder hump. Can grow till 15 cm.