Goldfish

People usually start searching about goldfish when they’re standing in a pet shop aisle, rescuing one from a tiny bowl, or wondering why a long-lived fish is suddenly floating, clamping its fins, or gasping at the surface. The stakes are simple: goldfish cope with a lot, but poor water quality and cramped housing shorten lives fast.

Below is the practical picture—how big goldfish really get, how long they can live, the water conditions that matter most, and the common problems that show up when something in the tank is quietly slipping out of balance.

Goldfish at a glance

• Scientific name: Carassius auratus¹
• Adult size: commonly 20–30 cm, but they can reach ~40–45 cm in the right conditions (especially pond fish)^{1, 2}
• Lifespan: often 10+ years with good care; some individuals live 20–25+ years^{1, 2}
• Diet: omnivorous—plant material and small animals/invertebrates²
• Temperament: generally social and often kept in groups, with enough space and filtration to match their waste load¹
• Habitat (pets): aquariums and ponds; never release into waterways^{6, 7}

A quick note on the “23 cm goldfish” myth

Many care guides repeat a neat maximum length (often around 23 cm). In reality, goldfish have indeterminate growth and can keep getting larger when given space, oxygen, and stable water quality. In tanks, 20–25 cm is common; in ponds and the wild, much larger fish are well documented.^{1, 2}

History: from Chinese ponds to backyard tanks

Goldfish were domesticated from carp relatives in China, with selective breeding producing the familiar orange and the many “fancy” body shapes seen today. They later spread to Japan and Europe, becoming one of the world’s most widely kept ornamental fish.²

Types of goldfish (and why it matters)

Goldfish are often grouped into two broad styles, and they don’t behave the same in a tank.

Single-tail, fast swimmers

Common, comet, and shubunkin types are streamlined and active. They need more swimming room, stronger oxygenation, and careful planning if kept indoors because they grow large and produce heavy waste.^{2, 4}

“Fancy” goldfish

Fantails, orandas, ranchu and other fancy varieties are rounder-bodied and less efficient swimmers. They can struggle in strong currents and are more prone to buoyancy issues when diet or water quality slips.⁴

Housing: what goldfish actually need

Goldfish are sold as beginner pets, but their biology pulls in the other direction. They eat a lot, excrete a lot, and rely on you to keep ammonia and nitrite at zero through filtration and water changes.⁵

Tank size (a realistic baseline)

A small bowl can’t hold stable water chemistry, and it can’t support a proper filter. RSPCA Victoria notes fish bowls are not adequate and a filtered tank should be used instead.⁸

• Start bigger than you think. For one goldfish, many experienced keepers treat ~75 L+ as a practical starting point, then scale up with each additional fish (especially for single-tail varieties).
• Plan for adult size. Buying a “small” goldfish and upgrading later often means months of stress on the fish (and recurring water quality problems) before the upgrade happens.

Filtration and oxygen

Goldfish thrive in clean, oxygen-rich water. Use a filter sized for the tank (often oversized), keep media maintained without stripping beneficial bacteria, and ensure good surface agitation for gas exchange.⁵

Substrate and decorations

Choose smooth surfaces. Avoid sharp ornaments and gravel small enough to be mouthed and swallowed. Goldfish forage constantly; anything they can fit in their mouth will be tested.⁵

Water requirements: focus on stability

Goldfish tolerate a fairly wide range, but they do poorly with sudden swings—especially in temperature and pH. Aim for steadiness rather than chasing a perfect number.

• Temperature: broadly 4–25 °C is often cited as a workable range; many home aquariums sit comfortably around the high teens to low 20s depending on variety and season.³
• pH: keep it stable; a commonly recommended window is around 6.0–8.0.³
• Ammonia and nitrite: always zero (these are toxic, even at low levels).^{3, 5}

Feeding: less romance, more routine

Goldfish are omnivores and do best on a varied diet. The quiet rule is restraint: overfeeding is the most common path to chronic water quality issues and digestive trouble.

• Feed small amounts they finish promptly, once or twice a day.
• Use a quality staple (pellets or gel food), then rotate in vegetables and occasional higher-protein treats.
• Remove leftovers—food rots fast, and goldfish waste is already heavy.

Reproduction: what to expect if they spawn

Goldfish usually spawn in warmer seasons. Eggs are scattered and may stick to plants; hatching often occurs roughly within a week, depending on temperature.^{2, 9}

In community tanks, most eggs and fry are eaten. If you intend to raise fry, plan separate rearing space, gentle filtration, and a feeding regime suited to tiny mouths.

Common health issues (and what they often mean)

Most goldfish “diseases” begin as husbandry problems: rising ammonia, unstable temperature, overcrowding, or chronic stress. Treat the water first, then the fish.

Buoyancy problems (“swim bladder” signs)

Floating, sinking, rolling, or struggling to stay level can be linked to constipation, gulping air while feeding, sudden diet changes, infection, or chronic water quality stress. Fancy varieties are more prone because of their body shape.¹⁰

White spots and flashing

Salt-grain spots or scraping against objects can point to parasites such as ich. A thermometer and test kit usually tell the rest of the story: parasites take advantage when fish are stressed and water is unstable.

Fungal and bacterial infections

Cottony patches, ulcers, or fin rot are often secondary infections that bloom after injury or prolonged poor water quality. Improving conditions is not optional—it’s the foundation of recovery.

Goldfish and the environment: don’t release them

In Australia, released goldfish can establish in waterways and contribute to ecological harm. Queensland Government guidance explicitly lists goldfish among ornamental species that must not be released into waterways, and NSW DPI also treats dumping or releasing pet fish as illegal and harmful.^{6, 7}

If you can’t keep your fish, rehome it through a pet shop, aquarium group, or another keeper—don’t “set it free”.⁷

Final thoughts

A goldfish kept in clean, stable water with room to grow becomes a very different animal from the one seen in a bowl—active, long-lived, and quietly busy from morning to night. Most success comes down to three habits: a tank sized for the adult fish, filtration that can keep up, and a steady hand with feeding and water changes.

References

1. Animal Diversity Web (University of Michigan) — Carassius auratus (Goldfish): size, behaviour, lifespan
2. Encyclopaedia Britannica — Goldfish: domestication, types, size, lifespan, breeding
3. OATA (Ornamental Aquatic Trade Association) — How to look after goldfish in an aquarium/fish tank (water parameters)
4. Aqueon — Goldfish care guide (size, temperature ranges for varieties)
5. PetMD — Goldfish care sheet (water testing, temperature guidance)
6. Queensland Government — Identifying invasive freshwater animals (includes goldfish: do not release)
7. NSW Department of Primary Industries — Aquarium owners and trade (responsible ownership; illegality of dumping/releasing)
8. RSPCA Victoria — Fish: setting up an aquarium; fish bowls not adequate
9. NSW DPI — Goldfish (pest fish): reproduction timing and general biology
10. Merck Veterinary Manual — Disorders and diseases of aquarium fish (buoyancy/swim bladder signs and contributing factors)