Goldfish

Goldfish（圖片來源：Ry362，採用 CC BY-SA 3.0 授權）

Goldfish (Carassius auratus auratus) is an ornamental fish derived from a mutant lineage of the wild crucian carp (Carassius auratus) native to China. Through deliberate selective breeding and long-term domestication by humans, this lineage gradually developed into the ornamental form known today as the goldfish. Historical records indicate that people were already keeping these fish during China's Northern and Southern Dynasties period, although large-scale cultivation did not become widespread until the Song Dynasty. Over more than a thousand years of artificial selection, numerous morphological and color varieties gradually emerged. Today, Japan, China, Europe, and North America collectively maintain hundreds of distinct goldfish strains. These range from slender forms that closely resemble the ancestral crucian carp to rounded varieties with missing dorsal fins, prominent head growths, or even protruding eyes, illustrating the remarkable diversity achieved through ornamental fish breeding.

In the wild, crucian carp inhabit freshwater environments and commonly occur in rivers, lakes, ponds, and slow-moving waters. They tolerate low dissolved oxygen levels and turbid conditions better than many other fish species. Their feeding habits are omnivorous: they consume aquatic plants and algae but also feed on insects, water fleas, chironomid larvae, and small crustaceans. Crucian carp frequently forage by stirring up bottom substrates in search of food, which can increase water turbidity and influence nutrient cycling within the ecosystem. Wild individuals typically reach around 20 cm in length, although the maximum recorded size can reach approximately 45 cm. Their lifespan in natural environments averages 20 to 30 years. Under captive conditions, however, limitations in tank space and feeding conditions usually reduce life expectancy to roughly 10 years.

The life cycle of goldfish follows a seasonal pattern. During late spring and early summer, when water temperatures rise above 10°C, females deposit adhesive eggs on aquatic plants or other substrates. Males simultaneously release sperm nearby, resulting in external fertilization. Fertilized eggs usually hatch within 2 to 9 days. Newly hatched larvae initially rely on nutrients stored in their yolk sac before beginning to forage independently. Juvenile fish are typically dark in coloration at first, gradually developing red, white, or golden pigmentation over several months. If ornamental strains are released into the wild (an action that should never occur), their coloration often gradually reverts toward the brownish tones typical of crucian carp within a few generations. Goldfish are naturally gentle and tend to move in groups. They communicate with one another through chemical signals, tactile cues along the body surface, and vibrations in the surrounding water. Clear social hierarchies are generally absent, and they are typically peaceful toward both conspecifics and other fish species.

Carassius auratus（圖片來源：H.T.Cheng，採用 CC BY-NC 4.0 授權）

Pearlscale（圖片來源：Lerdsuwa，採用 CC BY-SA 3.0 授權）

Bubble Eye（圖片來源：Lerdsuwa，採用 CC BY-SA 4.0 授權）

Housing Requirements in Captivity

When keeping goldfish at home, the most important requirement is providing a stable and spacious aquatic environment. Although traditional imagery often depicts goldfish living in small bowls or glass globes, such containers are actually far too small and frequently lead to deteriorating water quality and deformities in the fish. Each adult goldfish requires at least 45 liters of water. For example, keeping four fish requires an aquarium of at least 180 liters, equipped with a strong filtration system. Goldfish possess high metabolic rates and produce substantial waste. Without effective filtration, ammonia and nitrite accumulate rapidly and threaten the health of the fish. Therefore, aquarists must establish a stable colony of beneficial bacteria capable of breaking down metabolic waste.

Goldfish are coldwater fish, and their most comfortable temperature range lies between 15°C and 24°C. A heater is generally unnecessary. When water temperature drops below 5°C, goldfish enter a dormant state in which activity and feeding decline significantly. Conversely, prolonged exposure to temperatures above 28°C can disrupt metabolism and weaken immune function. Partial water changes should typically replace one-tenth to one-third of the aquarium volume each week, and tap water should be treated with a dechlorinating agent beforehand. Sudden changes in water parameters—especially temperature shifts exceeding 5°C—can cause severe physiological stress, so water replacement should be gradual.

Substrate materials such as gravel or small pebbles can be placed at the bottom of the aquarium. These surfaces provide habitat for beneficial bacteria and allow goldfish to engage in natural foraging behavior. Aquarium decorations should avoid sharp edges to prevent injury to their delicate fins. Aquatic plants are beneficial because they absorb excess nitrogen compounds and provide hiding places or spawning surfaces. However, goldfish frequently nibble on plants, so hardy species such as hornwort or water lilies are preferable for long-term planting. In terms of stocking density, goldfish are best maintained in groups, as solitary individuals may experience stress.

Feeding Conditions

Goldfish are omnivorous and have a broad dietary range. In captivity, the most convenient feeding option is specially formulated goldfish food, such as flakes or pellets. These diets typically contain lower protein levels and higher proportions of starch and fiber, aligning with the species' ability to utilize carbohydrates. Feeding should be carefully controlled. One to two feedings per day are sufficient, and each feeding should provide only the amount of food the fish can consume within five minutes. Uneaten food rapidly degrades water quality and should be avoided. Goldfish often appear perpetually hungry, but excessive feeding can lead to obesity, digestive strain, and rapid deterioration of water conditions. Responsible feeding discipline is therefore essential.

To maintain vibrant coloration and overall health, additional fresh foods may occasionally supplement the main diet. For example, peeled boiled peas can assist intestinal motility and help prevent constipation. Softened spinach, bok choy, or lettuce leaves provide vitamins. High-protein foods such as bloodworms, brine shrimp, or water fleas may be offered occasionally to support growth and reproduction. However, these animal-based foods should not be given excessively, as overconsumption may lead to digestive issues. All live foods should also be checked to ensure they are free from parasites or pathogens.

Newly introduced fish often experience environmental stress and may refuse food. It is generally advisable not to feed them during the first three days after introduction in order to prevent uneaten food from accumulating and decomposing. Juveniles and adults have different nutritional requirements. Young fish require smaller food particles, which can be produced by grinding adult fish food into finer powder. Observing goldfish feces is also a useful indicator of dietary balance and health. Long, trailing fecal strands may suggest incomplete digestion and indicate the need to adjust dietary composition.

Breeding Conditions

Goldfish breeding in captivity is not particularly difficult but does require a dedicated breeding environment. Spring is the primary reproductive season. When water temperatures reach approximately 19°C, mature males develop small white breeding tubercles on their pectoral fins and gill covers and begin actively chasing females. Once eggs inside the female have matured, chemical cues are released that stimulate spawning behavior. The female deposits thousands of eggs on aquatic plants, where external fertilization occurs.

Without preparation, most eggs will be consumed by the parent fish within hours. To achieve successful breeding, aquarists must transfer the egg-bearing substrate into a separate tank and provide gentle aeration to prevent oxygen depletion. Goldfish eggs are approximately 0.1 cm in diameter. Viable eggs appear transparent, whereas eggs that turn milky white are dead. After about two to three days, larvae hatch and initially remain attached to tank walls or plants, relying on yolk sac nutrients for survival. After three to four days they begin free swimming, at which point they should be fed powdered food or newly hatched brine shrimp.

Throughout this process, water quality management is critical. Even slight increases in ammonia or nitrite can cause large-scale mortality among larvae. Strong aeration should be avoided, as excessive water turbulence may damage the delicate bodies of the young fish or cause deformities. Juvenile goldfish grow rapidly during their first year and may reach around 6 cm in length. Growth slows thereafter, and full maturity usually requires three to four years. Hybridization between different ornamental strains can produce unstable or malformed traits, so breeding attempts are best undertaken by experienced aquarists.

Preventing Invasive Spread

Goldfish are ornamental fish with a long history of human cultivation. They originated through centuries of selective breeding from crucian carp and therefore do not exist as a natural species in the wild. When abandoned or deliberately released into natural environments, they can cause severe ecological damage. In North America, Europe, Australia, and even parts of Africa, feral goldfish populations have already formed as a result of release or abandonment.

Goldfish reproduce rapidly and can quickly expand their populations. Their habit of foraging by disturbing bottom sediments increases water turbidity, which reduces light penetration and interferes with photosynthesis by native aquatic plants. They may also prey on amphibian eggs and insect larvae, while simultaneously competing with native fish species for resources and habitat. Even more concerning is their potential role as carriers of parasites and diseases, including anchor worm disease, fish lice, and koi herpesvirus. Once introduced, these pathogens can pose serious threats to aquaculture industries and native aquatic organisms.

For these reasons, goldfish owners must follow the principle of never abandoning pets and never releasing them into natural waters. If continued care becomes impossible, alternative solutions such as rehoming, adoption, or consulting local aquarium stores should be considered. Respect for ecological balance and respect for life are fundamental responsibilities of all pet owners.

Author: Shui-Ye You