Choosing a goldfish tank

A goldfish tank is bought once and lived with for years, and almost every other decision, from the filter to the number of fish, follows from it. So it is worth being slow here. The choices below are in order of how much they matter.

Footprint before height

Goldfish are horizontal animals. They patrol the length of a tank, turn at the ends and rarely make use of depth beyond the top 30 or 40 cm. Oxygen, meanwhile, enters at the surface. Both facts favour the same shape: for any given volume, the tank with the largest footprint, length times front-to-back depth, is the better goldfish tank.

Footprint beats height

Tall and narrow: 40 × 40 × 62 cm, about 100 L (nominal) Surface area 0.16 m². Less gas exchange, awkward to clean.

Long and wide: 80 × 35 × 36 cm, about 100 L (nominal) Surface area 0.28 m². More swimming length and more surface for the same volume. Better oxygen exchange, easier to filter evenly.

Keepers’ rule of thumb is a front-to-back depth of at least 40 cm (16 in), enough for an adult fancy goldfish to turn comfortably and a single-tail to turn at all; it is a convention, not a measured limit. Length is the next priority. Height comes last; a water depth of 35 to 45 cm (14 to 18 in) is plenty, and some keepers of ranchu and other fancies prone to buoyancy trouble deliberately keep it shallower.

Measure the space, and the route

Two measurements people take and three they do not, and the missing ones are the expensive ones.

What everyone measures: the width and depth of the surface the tank will stand on.

What gets forgotten:

• Room to reach behind. A tank pushed against the wall makes the filter, the cables and the back glass unreachable. Leave five to ten centimetres.
• Height above the tank. You need room to lift the lid, get an arm in to the bottom, and lift out a filter. A tank under a low shelf is a tank you will maintain badly.
• The route from the door to the stand. Doorways, stair turns, the swing of a door into a hallway, the height of a bannister. A 120 cm tank is awkward in a way its dimensions do not convey, and a tank that does not fit through the door is a tank you do not own.

And two things to check about the position itself: that a socket is within reach without an extension trailing across the floor, and that the walk to a tap and a drain is short. The water change routine happens at this spot every week for a decade, and a long walk is a common reason it stops happening.

Tank placement, stands and floor loading covers the structural side, which for anything above about 200 litres is a real question rather than a formality.

Glass or acrylic

Glass is the default for a reason. It is cheaper at standard sizes, resists scratches from magnet cleaners and sand, does not yellow, and is easy to find second-hand. Its drawbacks are weight, since an acrylic tank of the same size is lighter, and a faint green tint in ordinary float glass that low-iron glass removes at a price. Glass can chip or crack if struck, and its silicone seams have a finite life.

Acrylic is lighter, clearer and far more impact-resistant, which is why very large and custom tanks are usually acrylic. It also scratches easily, from a grain of sand caught under a cleaning pad or a careless scraper. Light scratches can be polished out; deep ones cannot. Acrylic needs a stand that supports the whole base rather than just the edges, and it costs more at the sizes goldfish keepers usually buy.

Rimmed or rimless

A rimmed tank has a plastic frame top and bottom. The frame stiffens the glass, hides the seams, accepts a standard hood and lid, and lets the tank sit on its edges, which forgives a stand that is not quite flat. A rimless tank is bare glass with polished edges, often low-iron, and it looks superb. It needs a foam mat underneath and a dead-flat, level stand, it has no lid unless you buy or make one, and its open top invites evaporation and the occasional jumper. For a first goldfish tank, rimmed is the practical choice; rimless is a choice you make for the look, knowing what it asks.

Shapes to be wary of

Most tanks are rectangles and the exceptions are worth knowing about.

Bow fronts are fine. A bow front is a rectangle with a curved front pane, usually with a generous footprint; the curve distorts the view slightly from an angle and costs nothing that matters.

Cubes look striking but give less swimming length per litre than a rectangle. A 45 cm cube holds about 90 litres and offers a goldfish 45 cm to swim in, which a rectangle of the same volume beats comfortably.

Column and tower tanks are the worst case: tall, narrow, minimal surface area for gas exchange, and almost no length. They are the shape of a bowl scaled up, and the bowl page covers why that combination is the problem rather than the volume.

Corner tanks waste much of their footprint on the angle and are awkward to clean into. Not disqualifying, but rarely the best use of the money.

Cylindrical tanks share every problem of a bowl and add distortion.

Very deep tanks deserve a specific note for fancy goldfish. A fish with marginal buoyancy control works harder in deep water, and several breeds are kept deliberately in shallower tanks for that reason — floating upside down covers the fish this applies to. A water depth of 35 to 45 cm is plenty for any goldfish.

Standard sizes and what they suit

Tanks come in a handful of standard sizes, and the stocking positions used across this site map onto them fairly neatly: a first fancy goldfish needs 75 to 100 litres and each additional one 40 to 50 litres more; a first single-tail needs 150 to 200 litres and each additional one 75 to 100. The tank size guide explains where those numbers come from, and how many goldfish can I keep applies them.

Size and dimensions	Suits
60 cm — 60 × 30 × 36 cm, ~65 L (17 US gal)	A single young fancy, soon outgrown
20 gallon long (US) — 76 × 30 × 30 cm, ~68 L	A single young fancy, soon outgrown
80 cm — 80 × 35 × 40 cm, ~110 L (29 US gal)	One fancy
40 breeder (US) — 91 × 46 × 41 cm, ~171 L	Two fancies; an excellent footprint
100 cm — 100 × 40 × 40 cm, ~160 L (42 US gal)	Two fancies
120 cm — 120 × 45 × 45 cm, ~240 L (63 US gal)	Three or four fancies, or one single-tail
55 gallon (US) — 122 × 33 × 53 cm, ~213 L	Long but narrow and tall; the 75 gallon is the better buy
75 gallon (US) — 122 × 46 × 53 cm, ~297 L	Four or five fancies, or two adult single-tails
150 cm — 150 × 50 × 55 cm, ~410 L (108 US gal)	Single-tails long term, or a real group of fancies

Volumes are worked out from the dimensions, so they are gross; the gallons in the US names are nominal and differ from what those dimensions give. Subtract 10 to 15 percent for substrate, decor and the gap at the top; the Suits column applies the stocking rule above to that smaller figure. If your tank is not on the list, the aquarium volume calculator will turn its dimensions into litres.

Buying second-hand

Used glass tanks are the best bargain in the hobby, and most are sound. Check three things. The silicone should be clear or evenly coloured, firmly bonded and unbroken; brown, peeling or bubbled silicone means resealing or walking away. The edges and corners should be free of chips, especially along the bottom. And the glass should be free of deep scratches when you look across it toward a light.

Then test it. Put the tank on a flat surface outdoors or in a garage, fill it to the brim and leave it for a day or two, checking the seams with a dry paper towel. A tank sold as a reptile enclosure may never have been built to hold water, so treat it with more suspicion. Clean with vinegar and water for limescale, never with soap. If second-hand does not work out, our buying guide to tanks for goldfish covers new options. Buying second-hand takes the rest of the setup — the filter, the heater, the decor and what to ask the seller.

Starter kits and all-in-ones

A whole shelf in every shop, and the fair assessment is about the tank rather than the components.

What you get: a tank, a hood with a light, a small internal or hang-on-back filter, usually a heater, sometimes a net and a sachet of food. The individual parts are generally serviceable.

What is wrong with it: the tank is sized for the fish being sold in the shop rather than the adult it becomes, and everything else is matched to that tank rather than to a goldfish. So the filter is undersized twice over — once because the tank is small, and again because goldfish produce more waste than many of the community fish these kits are designed around.

If you have one already, it is not wasted:

• Replace the cartridge with sponge and ceramic media you rinse rather than throw away — the filtration guide covers why the cartridge is the colony.
• Add a sponge filter on an air pump alongside, which roughly doubles the biological capacity for very little.
• Keep the whole kit when you upgrade. It becomes a quarantine tank or a hospital tank, which everyone needs eventually and always at short notice.

And if you are buying now: a bare second-hand tank of the right size plus a filter, conditioner and test kit usually costs less than a kit of the wrong size, and it is the purchase you will not have to make twice. What keeping goldfish costs sets out the proportions.

The stand and the weight of water

A litre of water weighs a kilogram, so a 200-litre tank weighs well over 250 kg (550 lb) once you add the glass, sand and stones. A 400-litre tank is over half a tonne. That load needs a purpose-built stand or very solid furniture, and a floor that will carry it. For tanks above 300 litres or so, place the tank against a load-bearing wall and across the joists rather than along one. Tank placement, stands and floor loading works through the whole question — the loaded weight by size, how to find the joists, what makes a stand a stand, and the places a tank should never go.

The stand must be level in both directions and its top flat, because a twisted base stresses glass seams. Shim the stand, not the tank. Rimless and acrylic tanks want a thin foam mat between the tank and the stand.

Getting it home and into the room

Short, because it is the part that goes wrong once and expensively.

• Carry it empty, always, and on its base rather than its side.
• Never carry a tank by the top rim. The rim is a brace, not a handle, and lifting by it puts the load through the seams.
• One person per end, and a third for doors on anything large.
• Support the base of an acrylic tank fully while carrying, because acrylic flexes.
• Put it on the stand before you fill it. This sounds obvious and is worth saying, because a filled tank cannot be nudged, shimmed or straightened — a 200-litre tank in the wrong position is a full drain-down away from being in the right one.
• Check the stand is level first, in both directions, and shim the stand rather than the tank.

Braces. Many tanks above about a metre have a central cross-brace across the top, and some have a floating glass brace. These are structural. Do not remove one to fit a light or to reach in more easily; the tank is relying on it to resist the outward bow of a filled pane.

Lids and evaporation

A lid keeps fish in, dust and cats out, and slows evaporation, which concentrates hardness and nitrate in the water that remains. Goldfish are not jumpers by habit, but a startled fish or one being chased during spawning can clear an open rim. Glass lids cut out most evaporation; mesh lids trade some of that for better gas exchange and cooler water in summer.

Topping up is not a water change, and confusing the two is how a tank drifts. Evaporation removes water and leaves everything dissolved in it behind, so a tank topped up for months has steadily rising hardness, and nitrate that no amount of topping up will touch. Top up with dechlorinated water to keep the level right, and change water on its own schedule to take the dissolved load out. A tank that is only ever topped up will test worse every month while looking exactly the same.

The gaps are where fish actually go. A lid with a cut-out for a filter pipe, a heater cable or a feeding flap has an opening, and that opening is often the one a fish finds. Single-tails are the ones to worry about: commons, comets and shubunkins are fast and can clear a rim from a standing start, while a round-bodied fancy very rarely manages it. Spawning season is when it happens, because a chased female will take any exit there is. Blocking the gaps with a piece of plastic mesh costs nothing and is the whole of the fix.

A lid changes the tank’s temperature and its air. It traps heat, which is worth knowing in a July heatwave when a goldfish tank is already closer to the top of its range than it should be, and it slows the gas exchange that happens at the surface. Neither is a reason to go without one; both are reasons to prop it open in hot weather and to make sure something is moving the surface underneath it.

Buy once, if you can

The argument the whole page has been building toward, stated plainly because it is the decision that costs the most to get wrong.

The tank is the one purchase you cannot upgrade in place. A filter that turns out to be too small is replaced for modest money and the old one becomes a second filter. A light, a heater, a siphon — all of them are incremental. A tank that turns out to be too small has to be replaced entirely, along with the stand, and the old one is worth a fraction of what it cost.

And the upgrade is more disruptive than it sounds. It means a full tear-down: moving the fish, moving the media wet and quickly, resetting the substrate and decor, and a week or two of testing while the tank settles — cleaning a goldfish tank covers doing it without losing the cycle, and it is entirely manageable and entirely avoidable.

What does carry over when you do upgrade: the filter, if it was bought generously; the media, which is the important part; the heater, the siphon, the test kit, the decor and the substrate. So the money is not wasted — but the tank and the stand are bought twice.

The pragmatic version, since budgets are real: buy the largest tank the space and the money allow, second-hand if that stretches it further, and accept a smaller starting stocking rather than a smaller tank. One fancy in 150 litres is a better position than three in 75, and it is the one that does not need solving again in a year. What keeping goldfish costs sets out where the money actually goes, and the ideal goldfish aquarium shows what the finished thing looks like.

Where not to put a tank

Not in direct sunlight, which feeds algae and swings the temperature daily. Not beside a radiator or heater, or under an air-conditioning vent. Not in a hallway or beside a door where it will be bumped. Not on a windowsill, a floating shelf, a chest of drawers or a kitchen unit. Not far from a socket, or so far from a tap that the weekly water change becomes a hike. Once the tank is where it will live, the filtration guide and the ideal goldfish aquarium cover what goes in it.