Fancy Goldfish Tank Requirements: Volume, Filtration, Temperature and Buoyancy

Body types of fancy goldfish, how much water each fish needs, filtration and flow for a heavy waste load, unheated temperature range, swim-bladder problems in round bodies and safe tankmates.

What "fancy" means: body types

All goldfish are the same species, Carassius auratus, a cyprinid native to China that, according to Wikipedia, was first bred for colour more than 1,000 years ago. Selective breeding split the domestic forms into two groups. Single-tailed varieties such as the common goldfish, comet and shubunkin keep a long, streamlined body. Fancy varieties have a short, egg-shaped body with a double tail and often carry further modifications: the fantail and veiltail add long finnage; the ryukin adds a pronounced hump behind the head; the oranda carries a raspberry-like head growth called the wen; the lionhead and ranchu combine a wen with the absence of a dorsal fin; the telescope and black moor have protruding eyes; the celestial eye has upturned eyes and no dorsal fin; the bubble eye carries two fluid-filled sacs; the pearlscale is nearly spherical with raised, pearl-like scales.

These are not small fish. Practical Fishkeeping (PFK) gives adult sizes of 15–20 cm for the ranchu, up to 20 cm for the ryukin and 20–31 cm for the oranda, and notes that the wen takes one to two years to develop; Wikipedia reports that captive goldfish can live beyond ten years.

Telescope, celestial and bubble-eye varieties have limited vision or fragile eye structures; Wikipedia and PFK state that they should not be kept with rough, sharp or pointed objects, and Wikipedia excludes them from outdoor ponds for the same reason.
Wen-bearing varieties (oranda, lionhead, ranchu) need decor that cannot abrade the soft head growth.
The pearlscale is described by Wikipedia as very susceptible to constipation, and its raised scales are damaged by sharp objects.

Volume and stocking

Sources agree that fancy goldfish need far more water than their sale size suggests, though the figures differ. Wikipedia gives about 38 L (10 US gallons) per fantail-type goldfish and about 76 L (20 US gallons) per common or comet. Tropical Fish Hobbyist (TFH) states that even the smaller breeds need a 20-gallon (about 76 L) or larger aquarium. PFK, working from adult body sizes of around 20 cm, considers a footprint of at least 120 × 60 cm appropriate for three adult telescope-eyes long term and recommends three or four fancies rather than six in a tank of that class. Wikipedia calls it a myth that goldfish grow to fit their container, and TFH adds that a goldfish that stops growing is stunted, which is a sign of ill health and leads to deformity and early death.

Bowls are ruled out by every source: Wikipedia notes that they are prohibited by animal-welfare legislation in several municipalities and cause death, disease and stunting through low oxygen and very high ammonia and nitrite.

Filtration for a heavy waste load

Goldfish produce a large amount of waste both as faeces and through the gills, and Wikipedia warns that the resulting build-up of ammonia and nitrite readily kills them. TFH adds that fancy varieties are more sensitive to waste than single-tails, so nitrate must be kept in check.

Turnover: TFH gives a general rule of a filter flow rate at least six times the tank volume per hour. PFK uses generously sized external filtration with a large volume of mature biological media, and describes the traditional approach as extremely large water volumes, powerful filtration and frequent water changes.
Flow: PFK cautions that the flow should not be too strong, because fancy goldfish do not swim easily, and directs filter outlets upward so that surface agitation provides oxygen. Wikipedia likewise notes that surface agitation from a pump or filter is what allows oxygen to dissolve.
Water changes: PFK carries out two to three one-third changes per week.

Feeding drives the waste load. Wikipedia describes goldfish as opportunistic feeders that do not stop eating on their own, so overfeeding leads both to excess waste and, in fancy varieties with a convoluted digestive tract, to intestinal blockage.

Temperature

Goldfish are a coldwater species and can live in unheated aquaria; Wikipedia gives an optimum of 20–22 °C and states that fancy varieties are endangered below 10 °C and all goldfish suffer above 30 °C. PFK considers 22–24 °C ideal for growth without stress, reports that hardy single-tails tolerate almost 0–30 °C, and stresses that fancy varieties are less hardy and need greater stability: any change up or down should be gradual, and tanks should not stand near radiators or in direct sun. Warmer water holds less dissolved oxygen, so PFK advises continuous aeration at higher temperatures. PFK also states that no heater is required. A wider treatment of unheated systems is in the guide coldwater-aquarium-guide.

Temperature also changes the waste load: a PFK piece on mixing goldfish with tropical fish calculates that metabolic demand at 25 °C is more than 140 percent of that at 15 °C, so the fish eats more and produces more waste, and notes that a shift of more than 3 °C within 24 hours does more damage than a constant 25 °C.

Buoyancy and swim bladder problems in round-bodied types

Wikipedia's article on swim bladder disease names fancy goldfish as particularly susceptible. Affected fish float nose-down, drift to the surface or sink to the bottom and may show a distended belly or curved spine. Listed causes include physical abnormalities and inbreeding, constipation from overfeeding, intestinal parasites, low water temperature and bacterial infection. A veterinary review of swim bladder disorders in koi (PMC) explains the anatomy: cyprinids have a physostomous swim bladder connected to the foregut by a pneumatic duct, which lets gas in and out through the gut but also allows bacteria to ascend into the bladder. The review lists fluid accumulation, collapse, overinflation and herniation as the main disorders, and names breeding selection in "fancy" cyprinid varieties among the non-infectious causes.

Wikipedia lists feeding green peas, raising the water temperature slightly to aid digestion and isolating the fish in a bare tank as common first measures, and prevention as clean water, stable slightly warmer temperature and nutritious food. The disease is not contagious but can be fatal if the cause is not addressed. The related entries swim-bladder-disease and constipation-bloat cover diagnosis and treatment in detail.

Tankmates

The safest companions for fancy goldfish are other fancies of similar swimming ability. Wikipedia notes that commons, comets and other fast varieties can eat all the food before slower fish reach it, so goldfish should be housed by body type; PFK adds that telescope-eyes, because of their poor vision, should not be mixed with more active goldfish varieties. On tropical fish the editorial sources disagree: PFK argues against mixing on grounds of temperature, metabolism and the difficulty of feeding a goldfish enough in a warm tank, while TFH considers the blanket prohibition a myth because some tropical species tolerate the low-to-mid 20s °C.