The short answer
Goldfish are big, messy, long-lived fish, not bowl pets. As a working guide, a single fancy goldfish wants about 75 L, with roughly 40 L for each extra fancy. Slim-bodied single-tails — commons, comets, shubunkins — grow much larger and faster, so they really belong in a pond or a tank of ~150 L and up. More water isn’t a luxury here; it’s what keeps their heavy waste diluted and their oxygen high.
Why goldfish need so much room
Two things drive the numbers: size and waste. A well-kept goldfish reaches 15–30 cm and can live 10–20+ years, so you’re housing a large animal for a long time. They also eat and excrete constantly, producing far more ammonia than a tropical fish of similar length. Big volume dilutes that waste between water changes and gives the filter something to work with. Cramped fish stay small, sicken easily and rarely reach old age.
Single-tails vs fancies
- Single-tails (common, comet, shubunkin): fast, powerful swimmers that top 25–30 cm. A common goldfish or comet is best suited to a pond or a very large tank.
- Fancies (fantail, ryukin, oranda): rounder, slower and smaller, so they cope with tank life — but still need generous space and strong filtration.
Getting the tank right
Choose the biggest footprint you can — length and width matter more than height for swimming and gas exchange. Pair it with over-rated filtration (see do goldfish need a filter?) and a cool, stable temperature. Our aquariums hub and best large aquarium picks are a good place to start, and how to set up a goldfish tank walks through the full build.