Species · Goldfish & coldwater
Common Goldfish
Carassius auratus
A pond fish at heart — the single-tail goldfish reaches a foot long, lives for decades, and is not a tank (let alone bowl) fish long-term. Give it a pond or a very large tank.
Difficulty: IntermediateColdwater (goldfish) tank✓ Fact-checked care guide
Quick facts
| Adult size | ~30 cm / 12 in or more |
|---|---|
| Minimum tank | Really a pond fish — a very large tank (190 L / 50 US gal+) at minimum for one, sized to the fish |
| Temperature | 18–22 °C / 64–72 °F (coldwater — no heater; tolerates a wide range) |
| pH | 6.5–7.5 |
| Hardness | Neutral to hard (adaptable) |
| Filtration & flow | Strong, well-cycled filtration — a heavy waste producer |
| Diet | Omnivore — a quality goldfish staple plus vegetables; feed modestly |
| Social | Social; but each fish is a large bioload and needs serious space |
| Temperament | Peaceful and sociable |
| Lifespan | 10–20+ years |
Overview
The common goldfish (Carassius auratus) — the classic single-tail "comet" or "shubunkin" type — is the goldfish sold cheapest and housed worst. It is a fast-swimming, cold-water fish that grows to 30 cm or more and lives for well over a decade.
Put simply, it is a pond fish. Unlike the rounder, slower fancy goldfish, the common goldfish really needs a pond or a very large tank — it is emphatically not a bowl or small-aquarium animal.
Water & tank
Goldfish are coldwater fish (comfortable ~18–22 °C / 64–72 °F, no heater) but they are big, fast and extremely messy. The RSPCA sizes a tank to the fish — length at least 4×, width at least 2×, height at least 3× the adult body length — which, for a 30 cm fish, points to a pond or a very large tank.
As an absolute tank minimum for a single common goldfish, think ~190 L / 50 US gal and up, with strong, well-established filtration. Keep ammonia < 0.1, nitrite < 0.2, nitrate < 50 ppm (RSPCA), which takes volume and filtration to achieve with such a heavy waste load.
Feeding
An omnivore — a quality goldfish staple plus vegetables (blanched peas, greens). Feed modestly; overfeeding fouls the water fast and causes digestive problems.
Behaviour & tankmates
Peaceful, sociable and interactive — but keep common goldfish with their own kind (or in a pond), not with tropical fish (temperature) or small fish (which they eat). Every extra goldfish is a big extra bioload.
What to expect
A fast, active, long-lived fish that quickly outgrows any "starter" tank. Plan for its adult size and decades-long lifespan from the start — ideally a pond.
Myths, kindly busted
“Common goldfish are small fish for a bowl or little tank.”
They reach 30 cm+, live 10–20+ years, and are pond fish — a bowl or small tank stunts and shortens their lives.
“A goldfish only grows to the size of its tank.”
Stunting from too small a tank is harm, not a size limit — it damages organs and shortens life.
When to act
We triage, not diagnose — but these are worth a closer look (starting with a water test):
- Gasping at the surface or lethargy — almost always water quality with such a messy fish; test and do a water change.
- Being kept in a small tank at all — the biggest welfare issue for this species is simply too little space; upgrade to a pond or very large tank.
Sources & further reading
- RSPCA Knowledgebase — How should I care for my goldfish? — Tank sized to the fish (length ≥4×, width ≥2×, height ≥3× adult body length); bowls unsuitable; coldwater; ammonia < 0.1 / nitrite < 0.2 / nitrate < 50 ppm; lifespan 10–15 yr (some to 30). Common single-tails reach ~30 cm — effectively pond fish.
Written & fact-checked by The Finfern Team against the sources above — kind, general guidance, not a diagnosis. If something looks wrong, test your water and, if it persists, contact a fish-savvy vet.
Join the shoal
Honest freshwater care, the occasional myth kindly busted, and new guides — straight to your inbox. No spam, no gatekeeping.
Water-first tips and new guides the moment they land. No spam — unsubscribe anytime.