How many goldfish can I keep in my tank?

The fish shop will often say a 60-litre tank holds three or four goldfish. Experienced keepers will say it holds one, for now. The difference is not fussiness. It comes down to how much waste a goldfish produces, how large it gets, and what it takes to keep the water clean around it.

The ranges this site uses

For fancy goldfish (fantails, orandas, ranchus and the other round-bodied breeds), plan on 75–100 litres (20–26 US gallons) for the first fish and 40–50 litres (11–13 US gallons) for each additional one. For single-tailed goldfish (commons, comets, shubunkins), plan on 150–200 litres (40–53 US gallons) for the first fish and 75–100 litres (20–26 US gallons) for each additional one, and be aware that many keepers consider 280 litres or more, or a pond, the right home for adults.

Why a range and not a number? The lower figures assume a strong filter, weekly water changes and fish that stay at the smaller end of their adult size. The upper figures give you margin for a missed water change, a big fish, or a heavy-handed feed. Recommendations vary between keepers because their filters, habits and fish vary, and both ends of the range are defensible. The tank size guide explains where the numbers come from; this page applies them.

Worked examples, tank by tank

Tank	Fancy goldfish	Single-tailed goldfish
60 L (16 US gal)	One juvenile, with an upgrade planned	None long term
100 L (26 US gal)	One, comfortably	None long term
150 L (40 US gal)	Two	One
200 L (53 US gal)	Three	One
280 L (74 US gal)	Four; five with strong filtration	Two

Every figure in that table is for adult fish. That is a very important thing to know about it, and the reason it disagrees with almost every shop — see the table is for adult fish below.

60 litres

Below the first-fish figure for either type, but not useless. A single young fancy in a well-filtered 60-litre tank is in a far better position than in a bowl, and the tank will serve it for a year or so while you plan the next one. For a single-tail it is a temporary tank at best.

100 litres

One fancy, comfortably, for life. Two small fancies is a stretch: it sits just under the lower end of the range for a pair, and it works only with a filter turning the water over generously and a disciplined 40–50% weekly change. A juvenile comet or common can live here for its first months, but it will outgrow the tank.

150 litres

Two fancies with margin, and three only if the filter is generous and the water changes are never skipped. For single-tails, this is the lower end of the first-fish figure: one common or shubunkin, and plans for something bigger if it turns out to be a large one.

200 litres

Three fancies comfortably, four at the lower end of the range. One single-tail comfortably. A second single-tail pushes past the range for a pair; if two commons or comets is the goal, this is the tank to skip in favour of the next size.

280 litres

Four fancies with room to spare, five with strong filtration, and this is the size at which a group of fancies begins to look like a group. Two single-tails fit, which is why 280 litres is the volume many keepers treat as the sensible starting point for the streamlined breeds. Beyond this, the arithmetic continues in the same steps, and for three or more adult single-tails a pond starts to make more sense than an aquarium.

The table is for adult fish

Which sounds obvious and is the reason most overstocked tanks got that way.

A goldfish leaves the shop at three to five centimetres. A fancy reaches most of its adult size in two to three years, a single-tail goes on growing for several years more, and the mass goes up far faster than the length does — a fish that doubles in length is roughly eight times the animal, eating and excreting in proportion. The growth page has the year-by-year figures.

So four juveniles in a 60-litre tank are genuinely fine on the day they arrive. The filter keeps up. The water tests clean. Nothing looks wrong, and nothing will look wrong for months, which is exactly the problem: by the time the tank is obviously too small, the fish are too big to be anything but a difficult decision.

Stock for the fish you will have in two years, and accept that the tank will look understocked for the first one. That is what a correctly stocked goldfish tank looks like at the start.

Why the shop’s number is different

Worth understanding rather than resenting, because it explains most of the confusion on this subject.

The shop is answering a narrower question honestly. Four small goldfish do fit in a 60-litre tank on the day they are sold — the filter keeps up, the water tests clean, and the fish are fine. The answer is correct about that moment. It is simply not an answer about the adult fish, and nobody in the transaction is present two years later when the question changes.

Some shops give excellent advice, and the choosing guide covers how to tell. But where a shop’s figure and this page’s figure disagree, it is usually not a dispute about goldfish. It is two answers to two different questions, and yours is the longer one.

What being overstocked actually looks like

Most people arrive at this page because a number somewhere disagreed with their tank. Here is how to settle it with evidence rather than arithmetic.

Nitrate at the end of the week, tested just before a water change, is the single best measurement. The figures below are nitrate as NO3; a kit that reports nitrate-nitrogen shows about a quarter of them. It is what the fish produced minus what the tank removed, accumulated over seven days.

• Under 40 mg/L on a normal feeding schedule: the stocking is within what the water can carry.
• Above 40, or held down only by changing water twice a week: the tank is at or past its working limit, whatever the table says.

Four other signs, none of them conclusive alone:

• Fish at the surface in the early morning. Oxygen is lowest just before dawn, after a night of respiration with no photosynthesis. A tank that is fine at noon and gasping at six is a tank with no oxygen margin, which is what overstocking removes first. The gasping page covers the other causes.
• Growth that has stalled. A young goldfish that has barely changed in a year is being held back by its environment.
• Algae you cannot get ahead of. More fish means more nutrients, and algae is the visible part of that.
• Feeding time as a scramble. Fish that mob food and chase each other off it are usually short of space rather than short of food.

If you are already over

A lot of readers are, and usually through no fault of their own — a fairground fish, an inherited tank, or a shop’s advice followed in good faith. There are three options.

Upgrade the tank. The best outcome wherever the money and the floor space exist, and cheaper than people assume: second-hand tanks are plentiful, and the costs page sets out what a realistic upgrade involves. Move the filter media rather than the water, as the tank size guide describes, and the cycle comes with you.

Rehome some of the fish. This is a responsible decision and not a failure, and it is worth saying plainly because a lot of people will suffer an overstocked tank for years rather than admit to it. Aquarium societies, local fish shops, schools and keepers with ponds will often take healthy goldfish. Ask before you arrive, move the fish properly — the handling guide covers the method — and be open about the fish’s history.

Or manage it deliberately. Feed less and feed better, so less goes in; change water twice a week rather than once, which holds nitrate down more evenly than one large change; test weekly, because in an overstocked tank the margin between fine and not is measured in days; over-filter, which buys biological capacity; and write down a date for one of the first two options. A stopgap that is managed properly is a reasonable position. A stopgap that is a plan to think about it later is how the fish ends up living its whole life in it.

Does anything else count against the number?

Snails: barely. A handful of nerites or a few ramshorns add a small fraction of what one goldfish produces, and the snails page covers which ones survive being investigated by a goldfish. Treat them as free, within reason.

Plants: a little, in the right direction. They take up some nitrate and release oxygen during the day, both of which help. But the quantity of plants that would meaningfully change a stocking figure is far more than a goldfish tank usually holds, most goldfish eat anything soft-leaved, and plants consume oxygen at night rather than producing it. Plant the tank — the plants guide covers what survives a goldfish — because it is better for the fish and nicer to look at, not to buy room for another one.

Other fish: fully. Every coldwater tankmate comes out of the same budget. A plecostomus is the classic error — bought as a cleaner, it produces as much waste as a goldfish and reaches a comparable size. The tankmates guide covers what actually works.

Why bioload matters more than length

The old rule of one inch of fish per gallon was worked out for small, slim tropical fish and it fails goldfish badly. A 20 cm fancy is a deep, thick-bodied fish that weighs many times what a 20 cm slender fish does, eats accordingly, and produces waste in proportion. Goldfish also lack a true stomach; food moves through them quickly and more or less continuously, so they graze and excrete all day.

Why a pond’s arithmetic is different

Worth reconciling, because the figures look contradictory side by side. This page gives 150 to 200 litres for a first single-tail in an aquarium; goldfish ponds gives about 100 to 300 litres or more per adult single-tail, depending on filtration. Both are right, and the difference is the system rather than the standard.

A pond has far more surface per litre. Gas exchange happens at the surface, and a wide shallow body of water has vastly more of it per litre than a deep glass box — which is most of why the same fish is better oxygenated outdoors.

It carries a much larger biological surface. Liner, silt, plant roots and a pond filter together hold a colony no aquarium filter matches.

It buffers temperature, so the daily swing a small tank rides is largely absent.

And the fish eat far less for half the year. Below about 10 °C feeding stops entirely, so waste production falls away for months — feeding through the seasons covers the bands.

What has not changed is the measure. Clear water, zero ammonia and low nitrate decide whether a pond is overstocked exactly as they decide it for a tank. And one thing gets harder outdoors rather than easier: ponds spawn, so a pond stocked to its limit in April is over it by July — which breeding goldfish treats as a certainty to plan for rather than a risk.

Filtration and water changes are the other half

Every figure above assumes a filter that turns the tank volume over 5–10 times an hour with a good volume of biological media, and a weekly change of 25–50% of the water. Stock at the upper end and you are committing to the upper end of both. Our filtration guide covers what that means in practice, and the water change guide shows how to let nitrate readings tell you whether the tank is keeping up.

Shape matters too. Two tanks of 150 litres are not equal if one is long and low and the other tall and narrow. Goldfish swim horizontally, gas exchange happens at the surface, and a long footprint gives more of both.

Social needs: groups are good, one is fine

Goldfish are sociable by inclination rather than by necessity. In a pair or a small group they forage together, follow each other around and show more of their natural behaviour, and a group of fancies in a big tank is a genuinely lovely thing. Whether a single goldfish is lonely is not settled: there is little goldfish-specific research, RSPCA Australia advises keeping at least two where the tank allows, the RSPCA in the UK says goldfish do not need the company of other fish, and the Bristol Aquarists’ Society says they prefer it. What is clear is that a fish on its own in a clean, roomy tank does better than two in a tank that can only support one. Never add a companion to a tank that is already at its limit.

A spawn changes the arithmetic overnight

Rarely mentioned on stocking pages and worth planning for, because a healthy mixed group in a well-kept tank will eventually breed.

Goldfish spawn in spring, or after a temperature rise that imitates it, and a single female can release hundreds of eggs in a morning. Almost all of them are eaten within a day in a normal community tank, which is the outcome most keepers want and should not feel bad about — it is what happens in a pond too. The breeding page covers what spawning looks like so it is not mistaken for illness, because the chasing involved is vigorous enough to alarm anyone seeing it for the first time.

Two consequences for stocking. During the spawn, males pursue females relentlessly for days, and a tank that is adequate at rest can be short of the room a harried female needs to get away; a planted corner or a piece of hardscape she can break line of sight behind matters more than the litres do. After it, the tank is briefly carrying a large load of decomposing eggs and milt, which is a genuine ammonia spike — test for a few days and change water on the readings.

And if you do decide to raise a batch, understand that you are committing to a second tank and a stocking problem measured in dozens. Fry are easy to hatch and difficult to place.

Do not mix fancies with single-tails

It is a poor combination, and most experienced keepers avoid it. Commons and comets are fast, boisterous eaters that reach food first and leave slow, round-bodied fancies underfed; they also prefer cooler water and, as they grow, can jostle and nip. Keep fancies with fancies and single-tails with single-tails, and among the fancies, keep the slowest and most fragile breeds together. A telescope or bubble eye is best housed with its own kind rather than with a quick, sharp-eyed fantail. The compatibility checker covers goldfish with other species; within goldfish, the rule above is most of what you need.

Start below the ceiling

Whatever the tank, start with fewer fish than it can hold. The filter grows into its job over weeks, and each new fish adds ammonia the bacteria have not yet caught up with. Add one or two at a time with a few weeks between, quarantine newcomers if you can, and let nitrate readings tell you when the tank is full. The figure in the table is the ceiling, not the target.

Two to three weeks between additions, and the last one is the risky one. A filter colony sized for two fish does not become a colony sized for three the day the third arrives; it takes a couple of weeks to catch up, as setting a tank up sets out, and during that window the tank is running on the margin it had. That margin is widest at the start and narrowest at the end, which is why the fish that tips a tank over is almost never the second one. If you intend to reach the ceiling, plan the last addition for a month when you are around to test daily, not the week before a holiday.

Nitrate between water changes is the reading that answers this, not ammonia. Ammonia at zero means the filter is keeping up today; it says nothing about how hard it is working. Nitrate climbing faster each week, so that the same schedule no longer holds it down, is the tank telling you it is at capacity — and it says so while every other reading still looks fine. Take one reading immediately after a change and another just before the next, and watch the gap rather than the number. The water parameter checker will interpret a set of readings, and the stocking checker will tell you where a given tank and fish count actually sit.

Leave room for two things, not one. The obvious one is growth: the table is for adult fish, and the juveniles in front of you are a third of the size they will be. The less obvious one is you. Nearly everyone who keeps goldfish buys another goldfish, usually within the first year, and a tank stocked to its ceiling on day one turns that ordinary want into either a refusal or an overstocked tank. Stocking at roughly two thirds of the figure in the table leaves both the fish and the keeper somewhere to go.