Goldfish Care Sheet
By "Ren" Weeks
Common Name: Goldfish
Scientific Name: Carassius auratus
Also Known As: Common goldfish, comet, sarasa comet, shubunkin, feeder, single tail
Often Confused With: Koi
Origin: Domesticated in Asia ~ 800 A.D., but wild ancestors prevalent across globe as feral/introduced pest (present in USA as early as 1600s). Goldfish are found in virtually all temperate freshwater bodies, particularly lakes and rivers.
Description: Single tailed goldfish come in three primary varieties, each with individual morphological differences created through selective breeding. The varieties are as follows:
+ Common Goldfish are similar in form to the ancestral wild carp from which they were bred. A large, robust fish that may attain lengths of nearly two feet (60cm) in a lake or pond, these fish have a long, thick, somewhat laterally flattened body. The head is blunt and lacks barbells, with laterally placed eyes and a flexible mouth. Fins are short, sturdy, and angular, much like their wild cousins. Color is highly variable; fry normally start out silver and most often develop some variation of gold coloration, but colors may include yellow, orange, white, black, gray, chocolate, blue-gray, or any combinations and variations thereof. Color and markings may change throughout the fish's lifetime.
Left: Young Common Goldfish ; Right: Young Wild Type Goldfish © DKimages
+ Comets (Incl. Sarasa) are similar in appearance to common goldfish but have long, flowing finnage and a somewhat sleeker build. They attain a similar length, sometimes somewhat smaller, but are not usually as girthy. The fins, particularly the tail and ventral fins, are long and flowing at maturity (and appear comically oversized when young). Variability of color is similar that of common goldfish; fancy markings are more likely due to selection of this species as an ornamental fish as opposed to a feeder animal.
Left: Young Comet ; Right: Young Sarasa Comet © DKimages
+ Shubunkins are the smallest of the single tailed varieties and are seen as an intermediate between commons and fancies by many. They have long, flowing, ornamental finnage, a sleek build, softer facial features, and highly decorative coloring. Often referred to as "calico" or “koi” markings, these fish typically have a densely speckled body consisting of orange, white, black, and sometimes red markings. The often reach lengths of only one foot and as a result are a favorable alternative to koi. They are also the only single tail that could potentially be housed in a (large, several hundred gallon) indoor aquarium (as opposed to commons and commets, who are recommended for ponds and lakes only).
Maximum Adult Size: Commons and Comets up to 24" - 12-18" most common (60cm; 30-46cm); Shubunkins up to 1.5ft - 10-14" most common (46cm; 25.5-35.5cm)
Lifespan: Average 15-25 year potential lifespan, but accounts of lifespans up to 40 years have been recorded with proper husbandry. Due to poor care conditions, many live much shorter lives.
Suggested Care Level: Hardy, but demanding. Not for beginners.
Minimum Tank Size: Pond keeping is strongly suggested. Due to the high waste output, large size, social nature, and skittish behavior of single tail goldfish, most are ineligible for aquarium housing at adulthood. Grow-outs should be large and well filtered, and upgrades frequent; alternately, merely raise the fish in a pond to begin with. It is suggested to provide 3-5g per inch of adult length for proper growth, so at the absolute minimum, 50g+ (190+L) per goldfish should be offered to adults 10” (25.5cm) or larger. Large common adults would do better in 75-100g+ (285-380L) per fish. Understocking is always best.
Wintering Pond Fish: Please Note: if you live in an area where winter temperatures drop below freezing, the depth of your pond must be lower than the frost line. In most areas, this means ponds should be no less than three feet in depth. Aeration should be maintained throughout the winter via a strong bubbler, which will create a hole in the ice. You may wish to consider a trough heater for particularly harsh winters to prevent the pond from freezing solid. If your goldfish are living in a wide, shallow pond, you must provide indoor or moderately heated quarters, as such ponds will freeze solid - killing your goldfish in the process. You should cut down on feeding significantly when water temperatures reach 50-60*F (10-15.5*C) and stop feeding altogether when it reaches 40-50*F (4.5-10*C) in preparation for winter. Feeding sluggish goldfish in cold temps will cause a sharp decline in water quality and may result in death.
Planting/Decor: Goldfish are intelligent fish that do best in an enriched environment. This includes plants (plastic, silk, or live, but please note that goldfish will eat all but the hardiest of live plants), rocks, driftwood, hiding caves (a large terra cotta pot broken in half with its hole plugged is cheap and attractive), etc. Planting should be fairly dense but with room left for swimming. Sparse planting should be avoided if the fish are housed outdoors or are sexually mature as planting allows females to hide from male harassment and offers all fish a hiding place from predators.
Substrate: Goldfish may be kept on a variety of substrates. The most convenient is bare bottom, as waste can be siphoned easily. Sand and flourite are popular choices in grow-outs and indoor tanks but may cause cloudiness due to the "rooting" behavior of the species. A preferable alternative is large, rounded gravel ("river rocks"). Any gravel should be too large for the goldfish to fit in its mouth, or it presents a choking and impaction risk.
Filtration/Aeration Needs: Though goldfish can survive in a wide range of conditions in the wild, over-filtration and high aeration are preferred. It is generally recommended that you use at least 10x filtration for your goldfish. So, if you have a 100g tank, you want a filter performing at least 1,000ghp. Large canister filters or wet to dry filters are most often used for pond filtration. In a very well filtered pond with a large surface area, an airstone is not required. Goldfish should ONLY be kept in cycled ponds/tanks.
Temperature: Temperatures between 62-74*F (16.5-23*C) are best, however these fish are adapted for survival in cold climates and sustain life in ranges from 32-100*F (0-37*C). Long term exposure to high temperatures increases the incidence of disease and decreases the overall hardiness of the fish; staying to the cooler end of the spectrum is overwhelmingly preferred. Still, long term exposures to cold temperatures will promote a dramatic lull in metabolic processes that may result in prolonged fasting, lack of growth, and inactivity. See "Wintering" above.
pH: Neutral to slightly basic pH (7-8) is preferred. However, with appropriate acclimation, goldfish can thrive in 6.5-8.5.
Hardness: Moderate hardness is ideal for this species, but again, they can be acclimated to a wide range of water conditions.
Salinity: Tolerant of low salinity (~ 1tbsp/10g), but salinity is non-essential to health.
Diet: Goldfish are an omnivorous species that demand a wide range of food sources to remain healthy. The best approach to goldfish nutrition is to feed a pre-soaked high quality goldfish or koi pellet as a staple, or better still a gel based food, then supplement heavily with an abundance of fresh leafy green vegetables (aquatic or boiled terrestrial), plus a limited amount of vegetable, fruit, and live insect foods. Examples of good supplemental foods would be boiled dark green lettuces, many common water plants, blood worms, krill, peas, etc. Keep your eyes peeled for a “goldfish nutrition 101” pinned topic.
Temperament: Social, but not necessarily communal. Goldfish do well with and demand the company of other goldfish but are minimally compatible with community fish. This is due to their size, waste output, tendency to be a vector for parasites, and omnivorous nature. Young goldfish may be housed with other cold water species, but adults are best housed with only other single tailed goldfish. Very large individuals may be safely housed with Koi, but there is some risk of interbreeding (resulting in sterile hybrid offspring).
Suggested Tankmates: In general, only other single tailed goldfish. Juveniles may be housed with Rosy Red Minnows, Dojo/Weather loaches, or young Koi. Hillstream loaches are sometimes argued to be an acceptable tankmate, but their sensitivity to both parasites and waste may be problematic, as is their small adult size. Adult goldfish are best housed only with other goldfish or with Koi.
Sexing: This species has some limited degree of sexual dimorphism at maturity. Males tend to be less girthy and often smaller overall than females, with longer, sharper pectoral fins. By sexual maturity the pectoral fins often have a notably thick leading ray. The vent is narrower and more concave, and the gill plates will be covered with raised white bumps called breeding tubercles during mating season. The pectoral fins may also have these tubercles. Females, conversely, tend to be large and rotund with shorter, more rounded pectoral fins that lack the thickened leading ray. The vent will be wider and convex, and the flesh around the vent may become swollen during mating season. The white “bump” that females possess at the vent will protrude more during mating season as well. Vent sexing is most accurate and is depicted below:
© Red Arthur
Reproduction: Goldfish reach sexual maturity at roughly two years of age or ~5” if properly cared for, and are highly prolific breeders; if you have males and females in your pond, you will inevitably have births. It may become important to curb the population in one’s pond by removing eggs when spotted, rehoming fry, or culling, though adult goldfish and other predators often slow population growth.
In temperatures above 60*F (15.5*C), spawning is common, with spring and summer the most common months for breeding if housed outdoors. During this time period, male behavior becomes increasingly excitable and even aggressive, and breeding tubercles will form on the face and pectoral fins. The male will chase the female, nudging her vent with his head, to encourage the release of eggs. Because of this, one’s pond should be well planted in warm months particularly, as the female (and other males) can become badly stressed by the harassment.
There is no nesting behavior or prenatal & parental care in goldfish; the female expels eggs when stimulated by the male’s chasing, and the male fertilizes the eggs with milt (seminal fluid). Fertilized eggs typically appear clear with a small black “comma” within, whereas unfertilized eggs remain white. The number of 1-2mm eggs released varies significantly from 500 to 4,000 and averages about 1,000, though this number rarely survives to maturity due to predation. Typically, eggs are released in dense plants, as the parents will readily feed upon both eggs and young fry. Hatching is quick and typically occurs within 48 hours, slightly quicker or slower depending on temperature.
Newborn goldfish fry are only 3mm or so in length, translucent to gray with two large eyes and a full yolk sack. The fins are indistinguishable. After 48 hours or so, the yolk sac will be absorbed as the swim bladder forms, and the fry will begin to swim and seek food. They will consume infusoria and anything else they can fit in their mouths. Growth is rapid & dependent on the availability of food. Month-old fry, with adequate space and feeding, can reach up to one inch in length and will look like small, normally gray miniatures of adults.
Raising Fry (Deliberately): If one is deliberately attempting to breed goldfish, fertilized eggs or young fry should be brought inside and housed in a well-aerated grow-out tank. Crushed hardboiled egg yolk, infusoria, decapsulated brine shrimp eggs, vinegar eels, and commercial powders can be fed, three times daily up until the age of four weeks. Up until four months, feeding should be twice daily and can include pre-soaked or gel commercial diets providing the size is appropriate. Beyond four months, feeding can be reduced to once daily. It is very important to maintain clean conditions when caring for goldfish fry. At ages below one month, filtration (aside from a mild sponge filter after 48 hours) can not be utilized as the young fry are unable to fight the current and may be sucked into the filter. Thus, daily siphoning of uneaten food may be necessary. Watch water parameters closely as poor water quality can adversely affect development. Once the fry are over one month old, filtration can be added, but it is suggested that one place filter foam over the intake and a baffle over the output if the fish struggle with current.
Remember when breeding deliberately that pet store goldfish often yield from unhealthy, heavily inbred, mass-produced lines. If one seriously wishes to tackle breeding goldfish, he should seek out quality stock, or the chances of unhealthy or deformed fry is significant. The desirability of poor quality goldfish is low because they are so commonplace, which will make responsible rehoming difficult. Seek out a local aquarist’s society if you are interested in serious breeding to seek stock and get a feel for the desirability of single tail goldfish in your area. If it is minimal, you may wish to reconsider your endeavor.
Common Health Concerns: Goldfish are a hearty species, but a combination of poor care conditions in stores, poor breeding, and often improper husbandry has made illness run rampant among many specimens. The majority of health conditions are a response to water quality, so always test your water parameters with a functional test kit if your goldfish is ill. Ammonia burn, nitrate poisoning, ich, septicemia, swim bladder disorder, growth stunting, and egg binding are the most frequently observed conditions in both common and fancy goldfish. For more information on the causes, symptoms, and treatments of these conditions, please keep your eyes peeled for a pinned topic on common goldfish illnesses. A link will be added to this thread once it is complete.
Additional Notes/Comments: Goldfish are a spirited, intelligent, interactive, long-lived fish that can make an inexpensive and attractive addition to one’s pond. That being said, always research goldfish thoroughly before buying and ensure that you can provide for their lifelong care needs - including lifespan and space needs. This is an undeservedly oft-neglected species of fish. For additional information, please refer to and keep an eye out for pinned topics related to goldfish husbandry, including health, nutrition, and common myths.