04 Aquarium
The ideal goldfish aquarium
A walk through a goldfish aquarium built the way we would build it, wide rather than tall, filtered generously, planted with what goldfish leave alone, and easy to keep clean.
Every goldfish aquarium is a compromise between what the fish would choose and what fits in a home. This is the compromise we would make. It is not the biggest tank possible or the most expensive, and nothing in it is exotic. It is a set of decisions that, taken together, produce clear water, calm fish and a weekly routine short enough that you actually do it.
Anatomy of a goldfish aquarium
- Filter rated well above the tank’s volume, outflow softened for fancies.
- Air stone or surface agitation so oxygen can enter and carbon dioxide leave.
- Sand or bare bottom. Small gravel can be swallowed and lodge in the throat.
- Smooth decor with no sharp edges or gaps a fish could wedge into.
- Hardy plants such as anubias and java fern, attached to rock or wood.
- Thermometer where you can read it daily; avoiding large, fast swings matters more than the exact number.
- Lid and light on a timer for six to eight hours a day.
The tank: wide before tall
Goldfish swim horizontally. They cruise the length of a tank far more than they explore its depth, and the oxygen they use enters at the surface. Both facts point the same way: for a given volume, choose the tank with the largest footprint, not the tallest glass.
For two or three fancy goldfish the sweet spot is roughly 150 to 240 litres (40 to 63 US gallons), with a length of a meter or more and a front-to-back depth of at least 40 cm (16 in). A 120 × 45 × 45 cm tank holds about 240 litres and is the size many experienced keepers settle on for three or four fancies. Single-tailed goldfish need more of everything: a first comet or common needs 150 to 200 litres, and adults are happiest in 280 litres or more, or a pond. Our tank size guide covers the numbers for every combination.
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.
Glass or acrylic is a secondary decision. Glass is heavier and cheaper, resists scratches and is easy to find second-hand. Acrylic is lighter, clearer and stronger, but a sand-covered magnet cleaner will scratch it in one pass. For a standard-sized tank on a floor, glass is the default; choosing a goldfish tank weighs the trade-offs in detail.
The stand and the location
Water weighs about one kilogram per litre, so a 200-litre tank weighs more than 250 kg (550 lb) once glass, sand and stones are added. It needs a stand built for that load and a floor that can take it, which rules out floating shelves and kitchen units. The stand must be level and its top perfectly flat; a shim goes under the stand, never under the tank.
Location decides how much algae you fight and how stable the water stays. Keep the tank out of direct sunlight and away from radiators, heaters and drafty windows. Leave room above for your arms, and put it within reach of a power socket and a short walk from a tap and a drain. You will be carrying water to and from this spot every week for a decade; make the walk short.
Filtration sized for goldfish
Goldfish produce a lot of waste for their size, so the filter is sized for the fish rather than the tank. A common target is a turnover of five to ten times the tank volume per hour: a 200-litre tank wants a filter rated for 1,000 to 2,000 litres per hour. Rated flows are optimistic, so buy toward the upper end of the range.
The ideal filter combines a generous volume of biological media, where bacteria convert ammonia to nitrite and then nitrate, with coarse mechanical media that catches the solid waste goldfish produce in quantity. A canister filter suits tanks over 150 litres; a large sponge filter, or a hang-on-back with its cartridge replaced by sponge and ceramic media, suits smaller ones. Whatever you choose, the media must be rinsed in old tank water rather than under the tap, and the filter must be cycled before the fish arrive. The filtration guide compares the types, and cycling a goldfish aquarium covers the weeks before the fish.
How water moves through a filtered tank
- Intake with a sponge pre-filter. Catches debris before the media and protects fins, eyes and bubble sacs.
- The filter itself. Water passes through mechanical media first, then biological media where bacteria live.
- Gentle return. A spray bar or baffled outflow spreads flow so fancies are not pushed around the tank.
- Whole-tank circulation. Intake at one end and return at the other means fewer dead spots where waste settles.
Turnover Filter flow ÷ tank volume. A 200 L tank with a filter moving 1,200 L/h turns over six times an hour. Keepers commonly aim for roughly five to ten turnovers an hour.
Rated flow is optimistic Manufacturers quote flow with clean, empty filters. Real flow drops as media fills, so size up rather than exactly.
The outflow matters as much as the intake. Fancy goldfish, with their short, round bodies and long fins, are weak swimmers and tire in a strong current. The fix is to spread the flow: a spray bar along the back glass, an outlet pointed at a side wall, or a sponge over the outflow all keep the turnover high while softening the current the fish actually feel. Single-tails, by contrast, enjoy a little current.
Substrate
Goldfish spend their days sifting whatever covers the floor of the tank, taking mouthfuls and spitting them out. Keepers report small gravel lodging in a goldfish’s mouth or throat, and no study gives a safe or unsafe size, so gravel is off the list. The usual options are a shallow bed of fine, rounded sand, a bare glass bottom, or smooth stones too large to fit in the fish’s mouth.
This tank uses sand, about 2 cm deep (under an inch), which is shallow enough that waste sits on top where the siphon can reach it and the goldfish keep it turned. A few large, smooth river stones add visual weight. The substrate guide covers cleaning and the alternatives.
Planting that survives goldfish
Most aquarium plants are food or a toy to a goldfish. A short list is neither, because the leaves are tough and goldfish usually leave them alone: anubias in its many varieties, java fern, crinum (the onion plant) and bolbitis. Anubias and java fern are attached to wood or stone with thread or a dab of cyanoacrylate gel rather than planted, so there is nothing to uproot. Crinum goes into the sand with its bulb mostly exposed.
Two more tricks complete the planting. Pothos, the ordinary houseplant, grows with its roots trailing in the water and its leaves above the rim, so they stay dry, and goldfish rarely bite the roots. With its leaves in the air it can take up nitrate faster than a submerged plant under aquarium light. And floating plants such as duckweed and water lettuce, along with bunches of elodea, go in as food rather than decoration: the fish graze them steadily and you top them up. The planting guide has the full list of survivors and casualties.
Decor without sharp edges or traps
Goldfish are clumsy, and fancies especially so: telescopes and bubble eyes have eyes that catch on things, and orandas and fantails trail fins that tear on rough surfaces. So everything in the tank passes one test: run a finger over it, and if it would snag a stocking, it will snag a fin. Smooth driftwood, rounded stones and a terracotta pot with sanded edges pass. Resin castles with small openings do not, because a goldfish that fit through a hole in spring may be wedged in it by autumn. The decorations guide covers what to look for.
Light and photoperiod
Goldfish need no special light, and the plants on the survivor list are low-light species. A modest LED on a timer, running six to eight hours a day, is enough to show off the fish and grow anubias slowly. The timer is the important part: a consistent day and night keeps the fish’s rhythm steady, and it is the difference between a tank with a little algae on the back glass and one that turns green. There is more in aquarium lighting.
Aeration
An air pump driving an air stone or a sponge filter is cheap insurance. The bubbles themselves add little oxygen directly; their value is in breaking the surface and circulating the water so gas exchange keeps up. That matters most on warm days, when water holds less oxygen while the fish need more, and it keeps water moving if the main filter fails. The aeration guide explains how oxygen actually gets into the water.
Temperature and stability
Fancy goldfish do well at 18 to 24 °C (64 to 75 °F); single-tails tolerate a wider band, down to 10 °C (50 °F). What matters more than the exact number is avoiding large, fast swings. In most homes the room keeps the tank in range without a heater. In a cold or drafty room, a heater set low as a floor rather than to a tropical target stops the overnight swings that stress fish. A thermometer you can read at a glance belongs on every tank; see goldfish water temperature for the reasoning behind the ranges.
What this tank deliberately leaves out
As much a set of decisions as the list above, and arguably more useful, because most of it is on sale and none of it is needed.
No carbon dioxide injection. CO2 exists to push demanding plants, and the plants that survive a goldfish are low-light, slow-growing species that do not want it. It would also acidify the water and add another thing to fail.
No sump. A sump adds volume and hides equipment, and for a large system it is excellent. In a home it is plumbing, a flood risk and a pump that must never stop, for a benefit a canister filter already delivers. Sumps and large systems makes the case for the other side, and is specific about the size at which it starts to hold.
No ultraviolet clarifier. UV clears green water and knocks back free-swimming pathogens in the flow, which makes it genuinely useful on a pond. On a tank like this, clear water comes from the water change and the algae is managed by the light schedule. A lamp would be treating a symptom the design does not produce.
No chiller, because the room is the temperature control and a fan across the surface handles a heatwave.
No permanent chemical media. Activated carbon exhausts within weeks, does nothing about nitrate, and its main use is stripping a medication out of the water after a treatment. Running it constantly is a consumable habit rather than filtration.
No resin ornaments with holes in them, no coloured gravel, no bubble curtain, no plastic plants with cut edges. The decorations guide covers why.
And no tank mates. Not because nothing works, but because almost nothing does: the temperature range, the waste output and the speed mismatch rule out most community fish, and a goldfish eventually eats anything small enough. Goldfish tankmates covers the short list, and snails are the one addition this tank would actually accept.
The through-line is that most aquarium equipment is an improvement to a tank that already has the basics right. Footprint, filtration, water changes and restraint at feeding time do almost all of the work, and anything bought before those are in place is buying the wrong thing first.
The same tank, three other ways
The principles do not change with the budget or the room. Only the numbers do.
A first fancy tank, around 100 litres
The smallest version worth building. Widest footprint you can fit rather than the tallest glass; one hang-on-back filter with its cartridge replaced by sponge and ceramic media, outflow angled at the glass; a thin bed of sand; one piece of wood with anubias attached; a pothos cutting over the back rim; a light on a timer. One fancy goldfish, with 75–100 litres being the figure the tank size guide works to.
What you lose is margin rather than quality. A smaller volume concentrates waste faster, swings temperature faster, and forgives a missed week less. The design is identical; the tolerance is not.
A single-tail tank, 280 litres and up
The same thinking with everything scaled and one change of emphasis: current is welcome rather than something to soften. Commons, comets and shubunkins are strong swimmers that play in flow, so the spray bar can point along the tank rather than at the glass. Length matters more than ever — these fish reach 25–30 cm and want to cruise — and the floor loading genuinely needs checking at this size, because a 280-litre tank with stand and substrate weighs about 400 kg.
Two single-tails is a sensible ceiling here, and how many goldfish explains why the arithmetic gets unfriendly quickly. Beyond that the answer is a pond.
A bare-bottom tank, any size
What serious keepers and breeders often run, and it is worth understanding even if you would not choose it. No substrate at all: waste is visible, siphons out in seconds, and nothing can be swallowed or trap debris. Plants go in pots or attached to wood. It is the easiest goldfish tank in the world to keep clean and the least interesting to look at, and a goldfish deprived of sifting loses one of the few foraging behaviours a tank allows it — behaviour and intelligence covers what that costs.
A reasonable middle is what this page describes: a thin layer of sand, shallow enough to siphon easily and deep enough for the fish to work.
Where this tank would fail
Every design has a failure mode, and being clear about this one’s is more useful than pretending it has none.
Overstocking is the obvious one. Everything here assumes two or three fancies. Put six in it and the filter falls behind, nitrate climbs between changes, the plants cannot keep up, and the twenty-minute week becomes a constant negotiation. The design does not stretch; the arithmetic simply changes.
Skipping the water change is the second. There is no nitrate export in this tank beyond the plants and the siphon, and the plants are a rounding error against a goldfish’s output. Two weeks off is recoverable; two months is a different tank.
A sunny window would undo the light schedule. The timer controls algae precisely because the tank has no other light source. Put it where the sun reaches and the back glass becomes a green sheet and the temperature swings with the weather.
And a heavy hand at feeding time undoes all three at once, because every one of them is downstream of how much food goes in. That is the single lever with the most leverage, and it costs nothing.
None of these are exotic failures. They are the ordinary ones, which is the point: a tank built like this does not fail in interesting ways, it fails in the same three ways every goldfish tank does, and all three are habits rather than equipment.
The maintenance rhythm
The ideal aquarium is the one whose care fits into the gaps of a normal week.
Daily takes a minute: feed a small amount once or twice, watch the fish eat, glance at the thermometer. Weekly takes twenty to thirty minutes: change 25 to 50 percent of the water, hovering the siphon over the sand to lift waste, wipe the front glass, and top up the floating plants. Every few weeks, rinse the mechanical sponges in the bucket of water you have just removed, and test nitrate to confirm the schedule is keeping it comfortably under 40 mg/L and ideally under 20. The water change guide turns this into a routine, and the calculator below tells you what a 30 percent change of your tank actually is in buckets.
What it looks like after six months
By six months the tank has stopped looking new, and that is the point. The anubias has put roots over the wood and its rhizome has doubled in length. The crinum has thrown up leaves a meter long that trail across the surface. The back glass carries a soft green film that the fish graze and you leave alone. The sand has been pushed into ridges by a hundred afternoons of sifting, and the pothos has climbed a good way up the wall behind the tank.
The fish have grown, visibly, and their colour has deepened. They come to the front when you walk in. The water is clear from the front and from the side, the filter hums, and the weekly water change has become a habit rather than a chore. Nothing about this tank is remarkable, and that is the best thing that can be said about a goldfish aquarium.
Frequently asked questions
Do I need everything in this guide before I get a goldfish?
No. The essentials are a wide tank, a filter with enough turnover, a water conditioner and a test kit, and a cycled filter before the fish arrives. Plants, decor, a timer for the lights and an air pump can all come later. Many of the best goldfish tanks were assembled over a year, one improvement at a time, as the keeper learned what their fish needed.
Is a planted goldfish tank realistic?
Yes, with the right plants. Goldfish will eat or uproot most soft-stemmed species, but anubias, java fern and crinum have tough leaves that goldfish usually leave alone, and pothos grown with its roots in the water keeps its leaves dry above the tank while goldfish rarely bite the roots. Floating plants such as frogbit and duckweed get grazed, so treat them as a renewable crop. Expect some nibbling and a few floating leaves; the plants that survive the first month usually survive for years.
What would you leave out of this tank?
A great deal, deliberately. No carbon dioxide injection, because the plants that survive a goldfish do not need it. No sump, because a canister does the same job in a home with far less plumbing and no flood risk. No ultraviolet clarifier, because clear water here comes from the water changes rather than from a lamp. No chiller. No chemical media running permanently. No airstone curtain, no coloured gravel, no resin ornaments with holes in them. Most of what is sold for aquariums is an improvement to a tank that already has the basics right, and this one is mostly basics done properly.
Can these principles work in a smaller tank?
Yes, and the compromises are predictable. A 100-litre tank for one fancy goldfish uses exactly the same thinking — widest footprint available, generous filtration with a softened outflow, sand or bare bottom, tough plants attached to wood, a timer on the light — and simply holds fewer fish. What you lose is margin: a smaller volume concentrates waste faster, swings temperature faster and forgives a missed week less. The design does not change; the tolerance for error does.
How much does a setup like this cost?
It varies enormously by country and by how much you buy second-hand. The tank and stand are usually the largest costs, and used glass tanks are often the bargain of the hobby. A sponge filter and air pump are inexpensive; a canister filter is not. Buy the tank, filter, conditioner and test kit first, new or used as you prefer, and treat everything else as optional.