Goldfish are sold as starter pets, often in a small bowl or a tiny tank. Welfare groups say that is the wrong way round. A goldfish is a big, messy, long-lived fish, and it needs a home to match.
RSPCA Australia says plainly that traditional fish bowls are too small for goldfish and should be avoided. A goldfish can live 10 to 15 years on average, and some live far longer. Fancy goldfish (the round-bodied kinds with fancy fins) and common goldfish (the long, torpedo-shaped kind) both grow much bigger than the fish you see in the shop.
Goldfish eat a lot and produce a lot of waste. In a small volume of water that waste turns into ammonia, which is poisonous to fish, much faster than the filter's helpful bacteria can clear it (our nitrogen cycle guide explains how that works). More water means the waste is diluted, and the water stays steadier. A bowl has no room for a proper filter and very little surface for air to get in.
Growth is another thing. The SPCA in New Zealand notes that goldfish can reach about 45 cm for the common kind, and that poor water quality, not tank size alone, is what holds a goldfish back. A fish that stays small because its water is poor is not a fish that is fine.
There is no single number that every group agrees on, so here are two, and the calculator below uses the stricter one:
Both agree that goldfish need good filtration, and that a bowl is out. RSPCA Australia also suggests keeping at least two goldfish, because they are social. Start from the bigger figure if you are unsure.
Pick the kind of goldfish, how many you want to keep, and measure the inside of your tank in centimetres (length, width and the height of the water). The bars show your tank against the two rules.
Do not panic, and do not tip the fish into something unprepared. Plan an upgrade to a bigger, filtered, cycled tank (the water has to be ready first), and move the fish carefully. Until then, keep feeding small and keep the water clean. If your fish look unwell, gasp at the surface or clamp their fins, ask a vet who sees fish, or the shop where you bought it. Do not guess at treatments.
If you are getting a goldfish, ask a local rescue or fish society first, since unwanted goldfish are often rehomed.