Common goldfish

Quick facts

Adult size

30 cm

Lifespan

can live up to 25 years; well-kept goldfish routinely 15-25 years; oldest documented over 40

Tank zone

all

Temperament

peaceful

Difficulty

intermediate

Schooling

recommended 2+ (critical minimum 2, thrives at 3+)

Water parameters

Temperature

10–22°C

pH

6.5 to 8.5

Hardness

5 to 25 dGH

Tank requirements

Minimum volume

280 L

Minimum length

120 cm

Flow

low

Lighting

bright

Substrate

any

Open swimming room

needed

Feeding

Diet: omnivore, feeds primarily at the all.

Omnivore that eats constantly and produces waste proportional to its appetite. Sinking pellets are better than floating flake; goldfish that gulp air at the surface while eating can develop swim bladder issues. Feed 2-3 times daily, only what they consume in 2 minutes. Blanched peas (shelled) are the standard remedy for constipation and a good weekly supplement. Frozen bloodworm and brine shrimp for variety. In outdoor ponds they graze algae, mosquito larvae, and anything else they can find.

Vegetable matter required (algae wafers, blanched zucchini, spinach).

Compatibility

• Keep common goldfish with other common/comet goldfish only. Do not mix with fancy goldfish; commons are faster and will outcompete fancies for food.
• Not tropical. Do not house with tropical fish (tetras, gouramis, plecos). The temperature preferences barely overlap and goldfish waste output overwhelms filtration sized for tropical bioloads.
• Will eat anything that fits in their mouth, including small fish, shrimp, and snails. Adult commons have surprisingly large mouths.
• White cloud mountain minnows and weather loaches are the only common tankmates that share the cold-water preference and are large enough to not be eaten.

Habitat

Domesticated from the Prussian carp (Carassius gibelio) in China over 1,000 years ago. Wild-type coloration is olive-brown; the orange-gold color is a mutation selectively bred for. Common goldfish are the unmodified body type: slim, fast, and capable of reaching 30 cm in a pond. They are not tropical fish. They thrive in unheated water and are tolerant of temperature swings that would kill most tropical species. Outdoor ponds in USDA zones 4-10 can house them year-round as long as the pond is deep enough to avoid freezing solid (at least 60 cm depth). They produce enormous amounts of waste relative to their size, which is the main constraint on indoor keeping.

Breeding

Prolific egg scatterer that breeds readily in ponds when water temperatures rise in spring (above 20°C). Males develop white breeding tubercles on the gill covers and pectoral fins and chase females vigorously, nudging her flanks until she releases eggs. Eggs are sticky and adhere to plants and surfaces. Parents eat every egg they find. In an outdoor pond with enough plant cover, some fry survive. Indoor breeding requires a separate tank with spawning mops or dense plants, removing the adults immediately after spawning. Fry are brown-black for the first few months before developing orange coloration. Not all fry will color up.

Common problems

The single biggest problem is keeping them in tanks that are too small. A common goldfish in a 40 L tank is not going to thrive. They need 150 L minimum for the first fish, 50 L for each additional. Ammonia poisoning from inadequate filtration is the most common killer. Swim bladder disorder from constipation or poor diet causes the fish to float sideways or upside down; fast for 2-3 days, then feed shelled peas. Ich is common in new fish. Goldfish are cold-water fish and should not be kept with a heater. Fin rot from poor water quality is common in overcrowded setups.

Outdoor pond suitability

This species is suited to outdoor ponds, not just indoor aquariums.

Climate classification

cold-water

Outdoor pond zones (USDA)

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

Outside the zone range, this species can still be kept indoors. Within the zone, an outdoor pond at least 60 cm deep usually has enough thermal mass to overwinter the species, though local frost depth and surface freezing matter.

Bioload

Bioload coefficient: 8.0 (extremely high waste output; goldfish are the benchmark for 'messy' fish in the hobby).

Bioload coefficients are calibrated against the neon tetra as the anchor (1.0). See the methodology page for the formula and how each value was derived.

Further reading

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