Common goldfish

Water parameters

Tolerated range for this species. Aim for the middle of each band rather than the extremes.

Tank and habitat

Substrate: any.

Behavior

Plant interaction: destroys most plants.

Feeding

Omnivore with a heavy appetite and a waste output to match. Sinking pellets work better than floating flake: goldfish that gulp air at the surface during feeding can develop swim-bladder problems, particularly in fancy varieties but also in common-type fish under stress. Feed two or three small portions a day, only what the fish clears in about two minutes per feeding. Variety helps: dry goldfish pellets as a staple, with regular blanched (shelled) peas, blanched zucchini or cucumber, frozen bloodworm or brine shrimp, and occasional spirulina-based flake. Peas are also the standard constipation remedy when a goldfish floats sideways. In outdoor ponds the fish supplements its diet with algae, biofilm, mosquito larvae, and small invertebrates. Goldfish carry the enzyme thiaminase, so they should not be fed live to predatory fish (it destroys vitamin B1 in the predator).

Compatibility

• Common and comet goldfish are best kept with their own kind in groups of two or more, in a tank of at least 200 to 280 litres for the first fish or, more practically, in an outdoor pond
• Not tropical. Do not keep with tetras, gouramis, dwarf cichlids, plecos, otocinclus, or other tropical species; the temperature preferences barely overlap and the bioload mismatch overwhelms tropical-sized filtration
• Eats anything that fits in the mouth. Adults have surprisingly large gapes (3 cm or more); shrimp, snails, small tetras, small minnows, and goldfish fry all get eaten. Even mid-sized white cloud mountain minnows are at risk from full-grown commons
• Do not mix with fancy goldfish (orandas, ranchu, ryukin, telescope eyes, etc.). Commons are faster and tougher; fancies are slower with worse eyesight and will lose every feeding race
• White cloud mountain minnows and weather loaches share the cold-water preference and are workable companions only for small immature goldfish; once the goldfish reach adult size the minnows become food
• Outdoor pond compatibility: hardy submerged and floating plants (hornwort, anacharis, water lettuce, water hyacinth) get grazed and uprooted but generally regrow faster than the fish damage them

Origin and habitat

A domesticated form of Carassius auratus, originating in China from selectively bred crucian carp ancestors. A 2020 peer-reviewed PNAS paper (Chen et al.) outlines the timeline: red-scale mutations on normally gray or silver crucian carp were first recorded during the Jin Dynasty (AD 265 to 420); during the Tang Dynasty (AD 618 to 907) preferred colour phenotypes were selected and raised in ornamental ponds and water gardens; during the Song Dynasty (AD 960 to 1279) the yellow-gold variety became associated with the imperial family, and commoners were forbidden from keeping yellow fish. The Ming Dynasty (1368 to 1644) saw the move to indoor keeping, which allowed the survival of fancy varieties that would not have made it in outdoor ponds. The goldfish was carried to Japan in the late 1500s and to Portugal and the rest of Europe at the beginning of the 17th century, then to North America around 1850. Modern genetic work (Wang et al. 2013, peer-reviewed in PLOS One) places all Chinese goldfish lineages as descended from native southern Chinese C. auratus, particularly from the lower Yangtze River basin. The two commonly conflated single-tailed forms are different selective breeds within the species: the common goldfish has a short, slightly-forked tail and a moderately stocky body, while the comet has a much longer, deeply forked tail and a slimmer, faster body. Both are sold under the same scientific name, both are descendants of the same domestication event in the Yangtze basin (a 2009 study supports a single-origin model), and the trade often uses the names interchangeably even when the fish are visibly different. In outdoor ponds with adequate space the fish reaches 25 to 30 cm and occasionally 35 cm or more; indoor aquarium specimens often stunt to much less, partly from growth-inhibiting hormones the fish releases under restricted conditions but mostly from inadequate water volume and water quality. Lifespan is genuinely long: 10 to 15 years is typical in well-kept aquariums, 15 to 20 years in good ponds, and individual specimens of 30+ years are documented (the oldest reliable record is 49 years for a fish named Tish). The species is a cold-water fish, tolerant of roughly 10 to 22 C, and should not be housed with tropical species or heated to tropical temperatures. Released or escaped goldfish are an invasive problem worldwide, with established feral populations across North America, Australia, parts of Europe, and elsewhere, where the species can grow much larger than it ever does in captivity and outcompetes native species in many waterways. The Chinese name jinyu is a homophone for 'gold' plus 'surplus', which is part of why the species became an emblem of prosperity in Chinese culture. C. auratus is differentiated from other Carassius species by fewer than 31 scales along the lateral line (crucian carp have 33 or more).

Breeding

Prolific seasonal egg-scatterer that spawns readily in outdoor ponds when water temperatures rise above about 20 C in spring and early summer. Sexually mature males develop small white nuptial tubercles on the gill covers, head, and the leading edges of the pectoral fins; mature females become noticeably deeper-bodied as they fill with eggs. The males chase females through plant cover, repeatedly nudging the female's flanks to trigger egg release; eggs are sticky and adhere to plants, hardscape, and tank surfaces. Adults eat any eggs they can find, so spawning in an unprotected tank produces almost no fry unless cover is dense. In an outdoor pond with sufficient floating and submerged plants, enough fry survive to maintain or grow the population. For controlled breeding, place a conditioned pair or trio in a separate tank with spawning mops or fine-leaved plants, remove the adults as soon as eggs are scattered, and incubate the eggs in place. Fry hatch in three to seven days depending on temperature, are tiny but free-swimming after the yolk-sac is absorbed, and stay drab brown-black for the first few months. Adult coloration develops gradually over weeks to months, and not every fry colours up; the 'wild' brown coloration persists in some siblings. Sexual maturity is reached at one to two years.

Common problems

Inadequate tank size is the single most common cause of premature death. Common and comet goldfish need a real volume of water to live a normal life: rules of thumb in current hobby guides put a single fish at 200 to 280 litres with another 50 to 75 litres per additional fish, and a pond is genuinely better than any reasonable tank. Fish kept in small tanks (40 to 75 litres) typically die within a few years rather than the 15-plus they should reach. Ammonia poisoning from underfiltered tanks is the second leading cause of death; goldfish bioload is at the top of the freshwater hobby and undersized filtration cannot keep up. Swim-bladder disorder presents as the fish floating sideways or upside down and is most commonly caused by constipation, overfeeding, or rapid food intake with air-gulping at the surface; the standard response is one to three fasting days followed by shelled blanched peas. Ich is common in newly purchased fish from stressed shipments; heat treatment (raising the tank to about 27 to 30 C for 10 days) is safer than copper-based medication for goldfish, but heat shock from a sudden temperature increase is itself stressful, so move stepwise. Fancy varieties (orandas, ranchu, telescope eyes, etc.) are not appropriate tankmates for common or comet goldfish: the commons are faster, less fragile, and outcompete fancies for food, leaving the fancies progressively malnourished. Goldfish should not be housed with tropical species (the temperature ranges barely overlap), with sucker-mouth catfish (otocinclus and common plecos can latch onto goldfish slime coats), or with axolotls (mutual lethal interactions).

Outdoor pond suitability

Climate

cold-water

USDA zones

3–11 (winter low around -40°C or warmer)

Outdoor pond at least 60 cm deep for thermal mass. Local frost depth and surface freezing matter.

Bioload

Further reading

• How to cycle a fish tank
• Stocking ideas by tank size