Goldfish care: how long they live, what tank they need and why not a bowl

The goldfish is the most common pet fish in the world, and the one surrounded by the most misinformation. That it lives five years, grows to 12 centimetres, is happy in a bowl on the windowsill and remembers things for three seconds. None of that is true — and it is precisely these beliefs that cause most goldfish to die long before they should.

This guide gathers what veterinary and animal-welfare sources say about the goldfish: how long it really lives and how big it grows, how large a tank it needs and at what temperature, what and how much to feed, which fish it can share a tank with, and which signs tell you something is wrong. The figures come from the RSPCA, the MSD Veterinary Manual and VCA Animal Hospitals. It does not replace advice from a vet who treats fish.

How long a goldfish lives and how big it grows

Far longer and far bigger than most people think. The RSPCA notes that many believe a goldfish lives about five years and grows to around 12 centimetres — when in fact goldfish are known to live up to 25 years, and some reach over 30 centimetres. The short life most people have seen is not a trait of the species but a product of the conditions we keep it in.

So the only honest question at the shop is: where will this fish be in five years? The juvenile is three centimetres long; the adult may be fifteen to thirty. The tank has to be bought for the adult, not the juvenile.

Why a goldfish does not belong in a bowl

The round bowl is the most widespread and the worst container for a goldfish. The RSPCA is unambiguous: decorative or "fun" tanks such as goldfish bowls, jars and children's tanks have not been designed with fish welfare in mind. The reasons are concrete:

• A small volume gives no stable environment. In a few litres, temperature and water quality change quickly.
• There is no room for an efficient filter, and a goldfish produces a lot of waste.
• A small surface area means too little oxygen. Oxygen enters the water at the surface, and a bowl has the smallest surface for its volume.
• Temperature swings, which the RSPCA says cause stress and even death.

The MSD Veterinary Manual says the same from the other side: low dissolved oxygen and high ammonia are the two water quality parameters most likely to directly kill fish. A bowl without a filter offers exactly that combination.

How big a tank a goldfish needs

The sources differ on the exact figure, which is worth saying plainly — and what differs most is whether a rule is for fish in general or for goldfish in particular:

VCA's general rule gives about 15 litres for a 10-centimetre fish; the RSPCA's goldfish rule gives 40 — and the RSPCA states outright that goldfish need more space than tropical fish. Since an adult goldfish is 15 to 30 centimetres long, 60 litres is the floor for one fish, not a recommendation. The RSPCA also warns these are only guidelines, because water quality depends on much else: temperature, filtration, feeding.

Shape matters: the RSPCA stresses that surface area is vital, because that is where oxygen dissolves into the water. A long, low tank is better for a goldfish than a tall, narrow one of the same volume.

Temperature and equipment

The goldfish is a cold-water fish. The RSPCA gives a range of 10–21 °C and adds that a heater is not normally necessary; if you worry about swings, set one to the low end. That is also why a goldfish does not belong in a 25 °C tropical tank alongside guppies or neon tetras.

A filter is mandatory, not optional. VCA recommends running the tank with water and filter for 4 to 6 weeks before adding fish, until ammonia and nitrite are acceptable; the RSPCA calls this "fishless cycling" and recommends it explicitly. What happens during those weeks and how to measure it is covered in our guide to cycling an aquarium.

The full setup, from choosing a spot to the first week, is in our guide to setting up your first aquarium.

What a goldfish eats, and how much

Buy food made for goldfish. The RSPCA specifically warns not to feed goldfish tropical fish flakes, because they contain different levels of nutrients. Supplement flakes or pellets with fresh food: the RSPCA notes that goldfish love chopped vegetables such as peas and spinach.

Quantity matters more than type. The RSPCA recommends as much as the fish eat within two to five minutes, and two or three small feeds a day rather than one large one. Fish learn the routine and will be waiting at the glass.

Which fish can live with a goldfish

First: a goldfish does not need the company of other fish and, according to the RSPCA, can be kept on its own. If you want more fish, three rules apply.

Not with tropical fish. A goldfish lives at 10–21 °C; tropical fish need a heater. A shared tank would be wrong for one side or the other.

Not single-tails with twin-tails. The RSPCA warns not to keep single-tailed varieties with the natural shape (common goldfish, comet, shubunkin) together with twin-tailed "fancy" varieties: single-tails are faster, better swimmers, while twin-tails are slow and lose out at feeding time.

Not with koi in an aquarium. Koi are pond fish that grow far larger than goldfish and have no place in an indoor tank.

Cold-water species that share the goldfish's temperature range in practice — the White Cloud Mountain minnow, for instance — are described in our guide to the 10 best aquarium fish for beginners. The litres rule applies to them too: every extra fish needs its own space.

Goldfish varieties: common and "fancy"

All goldfish are one species, Carassius auratus, but centuries of breeding have produced very different shapes. The RSPCA sorts them into two camps:

For a beginner, the safest choice is therefore a common goldfish or a comet. Fancy varieties are beautiful but ask for more: steadier water, food that slow fish can reach, and a tank without faster tank-mates.

Maintenance: what, and how often

A goldfish produces more waste than most aquarium fish of the same size, so the maintenance schedule matters more than for other species. The sources differ slightly on the interval; the direction is the same:

What should the test show? Among its reference values for freshwater systems MSD lists ammonia 0 mg/l, nitrite 0 mg/l and pH between 6.5 and 9.0. Any measurable ammonia or nitrite means the filter is not keeping up with the load — or the tank is too small.

Signs that something is wrong

With a goldfish, the first diagnosis is almost always the water. Before you think of disease, test ammonia and nitrite.

MSD writes that most fish diseases result from stress, poor water quality, overcrowding and failure to quarantine new or sick fish. The signs people search for most, and what they mean according to the RSPCA and MSD:

Among preventive measures MSD particularly stresses quarantining every new fish before it joins the main tank. A goldfish from a shop has been in a crowded holding tank with dozens of others — a separate small tank for a few weeks is the cheapest insurance there is.

Three stubborn myths

"A goldfish has a three-second memory." No. The goldfish is one of the most studied laboratory models of spatial learning. In a study published in The Journal of Neuroscience (Rodríguez and colleagues, 2002), goldfish learned to find a goal in a maze from different starting points — which requires a map-like memory of space, and that memory depended on a brain region equivalent to the mammalian hippocampus. The fish waiting at the glass at feeding time is no coincidence.

"It lives five years and grows to 12 centimetres." The RSPCA names this belief and refutes it: up to 25 years and over 30 centimetres. What most people know is a goldfish that died early.

"A bowl is fine to start with." It is not. The RSPCA lists bowls, jars and children's tanks as unsuitable because they were not designed for fish welfare — and the reasons (volume, filter, surface area, temperature) do not depend on how long the fish stays in one.

Frequently asked questions

How long does a goldfish live?

According to the RSPCA, goldfish are known to live up to 25 years. The common belief in five years reflects the conditions most goldfish are kept in, not the capacity of the species.

How big a tank does a goldfish need?

The RSPCA gives at least 60 litres of water per goldfish, and the rule is per fish: two need 120 litres. Choose a long, low tank, because oxygen dissolves at the surface.

Can a goldfish live in a bowl?

No. The RSPCA explicitly lists bowls, jars and children's tanks as unsuitable: too little volume, no room for a filter, too little surface for oxygen and a temperature that swings.

Does a goldfish need a heater?

Not normally. The RSPCA gives a range of 10–21 °C; a heater only comes into play if room temperature swings, and then it should be set to the low end.

How much and how often should I feed a goldfish?

Per the RSPCA, as much as it eats in two to five minutes, two or three times a day. Use goldfish food rather than tropical flakes, and add chopped peas or spinach.

Can a goldfish be kept alone?

Yes. The RSPCA states that goldfish do not need the company of other fish. If you keep several, do not mix single-tailed and twin-tailed varieties, and do not add tropical fish.

Why is my goldfish lying on the bottom or swimming on its side?

The RSPCA describes this as a bad sign: it may be poor water, disease or a swim bladder problem. The first step is always to test ammonia and nitrite and do a partial water change; if it does not improve, see a fish vet.

Does a goldfish need a filter?

Yes. A goldfish produces a lot of waste, and according to MSD low oxygen and high ammonia are the most common direct causes of death. Per VCA, run the filter for 4 to 6 weeks before adding fish.

Does a goldfish really have a three-second memory?

No. A study in The Journal of Neuroscience (2002) showed that goldfish form map-like spatial memories and find a goal in a maze even from new starting points.

In closing

The goldfish is not an easy fish for beginners — it is a cheap fish for beginners, which is not the same thing. Give it what any other pet would get — enough room, a clean environment and moderate food — and it can be a companion for a decade or two. Give it a bowl, and you will repeat the story everyone knows.

The species profile with size and temperature data is on our goldfish page, and other species are in the aquarium fish directory. Related guides: setting up your first aquarium, the nitrogen cycle and 10 best aquarium fish for beginners.