04 Aquarium
Tank placement, stands and floor loading
A 200 litre goldfish tank loaded and running weighs around 270 kilograms, concentrated on the footprint of a small desk. Everything else on this site pushes you toward a tank that size; this is the page about where it can actually go.
Everything else on this site pushes you toward a bigger tank. The tank size guide asks for 75 to 100 litres for a first fancy and 150 to 200 for a single-tail, the stocking page adds more for each fish after that, and the growth page explains why the fish will need it.
What none of them says is what that weighs, or where a thing that heavy can actually stand.
The arithmetic
Water is a kilogram a litre, and that is only where it starts.
| Nominal tank | Water | Glass, substrate, rock, stand | Total, roughly |
|---|---|---|---|
| 60 L | ~55 kg | ~30 kg | 85 kg |
| 100 L | ~90 kg | ~50 kg | 140 kg |
| 150 L | ~135 kg | ~70 kg | 205 kg |
| 200 L | ~180 kg | ~90 kg | 270 kg |
| 350 L | ~320 kg | ~150 kg | 470 kg |
The working rule is about 1.4 kg for every litre of nominal volume, all in. It is an approximation and it is conservative in the right direction.
Where the extra comes from:
- The glass. A 200 litre aquarium weighs 30 to 45 kg empty, depending on whether it is float glass or low-iron, and on the thickness the size demands.
- The substrate. Aquarium gravel runs about 1.5 to 1.7 kg per dry litre. A 5 cm bed across a 100 × 40 cm footprint is 20 litres of gravel — around 30 kg, and it displaces water rather than adding to it, but it is heavier than the water it displaced.
- Rock and hardscape. Easy to underestimate. A single decent piece of slate or dragon stone is several kilograms.
- The stand itself, which is 20 to 40 kg of timber or steel before anything goes on it.
- Equipment. An external filter full of water and media is 5 to 10 kg on its own, usually sitting in the cabinet.
Which way the joists run
This is a very useful practical fact on the page, and it costs nothing to act on.
A suspended timber floor is a set of parallel joists spanning between supporting walls, with boards laid across them. A load placed along one joist is carried mainly by that joist. The same load placed across the joists is shared between several of them.
So:
Run the long axis of the tank perpendicular to the joists. This is free, and it spreads the weight across more joists.
Put it near a supporting wall, not in the middle of the span. A joist deflects most at its centre and least at its ends. A tank against a load-bearing wall is being carried almost directly by that wall.
Avoid the middle of a large room for anything substantial.
Finding the joists
- Floorboards run perpendicular to the joists. If you can see the boards, you know the direction immediately.
- The nail or screw lines in the boards mark where each joist sits.
- A stud detector works on floors as well as walls.
- Look at the ceiling below, if there is an accessible one — a cellar or garage shows the joists directly.
- Downstairs, a solid concrete floor removes the question entirely. Ground-floor solid floors will take anything a domestic aquarium can weigh.
When to ask someone qualified
A plain section, because this is the point at which a website should stop. The litre figures are rules of thumb, not engineering limits.
| Situation | Verdict |
|---|---|
| Any size, solid ground floor | No concern |
| Up to ~100 L, sound suspended floor | No concern; place sensibly |
| 100–250 L, upper floor | Placement matters. Across joists, near a wall |
| 250 L+, upper floor | Ask a builder or structural engineer |
| Any size, old or visibly deflecting floor | Ask, regardless of the number |
| Raised pond, balcony, roof terrace | Ask, always |
If a floor already bounces when you walk on it, or the building is old and you do not know what is under the boards, the size of the tank is not the deciding factor. Someone who can look at the joists is.
The stand
A purpose-built aquarium stand is not a marketing exercise. It is a frame that carries the load down to the floor around its perimeter and at its corners, with a top designed to support the tank’s base evenly. That is a different structure from a cabinet designed to hold plates.
Ordinary furniture is usually the wrong answer. A sideboard, a chest of drawers, a desk or a bookcase is built for a light distributed load. A quarter of a tonne concentrated on its top panel is outside what it was designed for, and the failure mode — a top panel bowing slowly over months until a seam goes — is the kind you do not see coming.
If you are reusing furniture anyway, the tests are:
- A solid top, not a thin panel over a void.
- A frame underneath, not four slender legs at the corners of a large span.
- A footprint at least as large as the tank’s base. No overhang, ever — an unsupported edge on a glass tank is a crack waiting for a reason.
- Flat. Run a straightedge across it.
Rimless and low-iron tanks are less forgiving than framed ones, because a framed tank transfers its load through the frame while a rimless one rests on the glass base itself. Those want a levelling mat — a thin sheet of high-density foam or polystyrene — between the tank and the stand, which takes up any small irregularity in the surface. Many framed tanks benefit from one too, and it costs very little.
Levelling
Under-discussed and responsible for a surprising share of tank failures.
A tank out of level does two things: it loads one seam more than the others, and it leaves part of the base unsupported. Both are slow failures. A tank can sit slightly out of level for a year and then split along a seam, and the cause will look mysterious.
Level the stand, not the tank. Spirit level across both axes, and shim under the stand’s feet with hard packers — not cardboard, not folded paper.
Check again after filling. Floors deflect under load, and a stand that was level empty may not be at 270 kg. Fill to a quarter, check, adjust if needed, then continue.
Check the stand is not rocking on an uneven floor before anything goes on it.
Noise, and the room it is in
Nobody mentions this until the tank is in a bedroom and running.
An aquarium is a continuous, low-level noise source, and the three sources are different problems:
The filter. A canister filter under the cabinet is nearly silent. A hang-on-back unit is not — the return trickling into the water is the loudest thing most tanks do, and it gets louder as the water level drops between top-ups. Keeping the level high closes the gap and quiets it.
The air pump. A small diaphragm pump makes very little noise by itself and a great deal once it is sitting on a wooden cabinet, which acts as a soundboard. Stand it on a folded towel or a block of foam, or hang it from a hook, and it effectively disappears. The aeration guide covers why you want one running in the first place.
The lid and the glass. A loose glass lid rattles against the frame in a room with a draught.
None of this is a reason not to have a tank in a bedroom or a study. It is a reason to know before rather than after, because a tank is not something you move to solve a noise problem.
Humidity, and the wall behind
An open-topped aquarium evaporates a surprising amount — several litres a week in a warm room, all of it into the air of that room.
Over a winter, in a small room with poor ventilation, against a cold external wall, that is enough to show up as condensation, then as damage to plaster or wallpaper, then as mould. It is a slow problem and it is entirely avoidable:
- A lid cuts evaporation dramatically and is worth having anyway, for the jumping reason if nothing else.
- A gap behind the tank lets air move across the wall rather than sitting against it.
- An internal wall rather than an external one, where you have the choice.
It is also the reason evaporation top-ups are a real part of the routine, and why they are a different job from water changes — the source water page covers why.
The flood you will actually have
Worth being specific, because the mental image is a tank shattering and the reality rarely is.
A sump that could not hold the drain-down. The one on this list that is decided at build time rather than by bad luck: when the return pump stops, everything above the overflow runs into the sump, and if there is not room for it the sump puts it on the floor. Sumps and large systems has the calculation and the test.
An external filter hose. A common cause of aquarium floods: a hose works loose at a connector, or a seal on the filter head fails, and the filter siphons the tank down to the level of its intake onto the floor. Check the connectors when you service it, and do not leave a filter running loose in a cabinet where a knock can shift it.
A siphon left running, or a hose left in the tank while a pump refills it. Human error, entirely preventable by not walking away.
An overflow during a water change. Also human error, also preventable.
A split seam or cracked base. Rare, slow to develop, and the thing all the levelling and support advice above is about. When it does happen it is usually traceable to an unsupported edge, grit trapped under the tank, or months out of level.
If it is happening now, stop the water before you touch the floor. For an external canister the pump is not what is moving it — the filter sits below the tank and siphons on the height difference alone, so switching it off changes nothing. Close the taps on the filter head, or if it has none, lift the intake out of the water; the flow stops immediately either way. A hang-on-back does stop when unplugged, because it cannot siphon below its own intake. A siphon hose stops when you lift the end above the water line. Only then start on the towels, and turn off anything electrical the water has reached, at the consumer unit rather than at the socket.
The practical mitigations are unglamorous: know where the stopcock is, keep a couple of old towels near the tank rather than in a cupboard upstairs, do not store anything under the tank you would mind losing, and service the filter rather than waiting for it to remind you. The cleaning guide covers the service interval.
Renting, insurance and the flat below
Three things worth checking once, rather than finding out afterwards.
Your tenancy agreement. Many mention aquariums specifically, and some set a volume limit. It is a five-minute read and it is easier to have that conversation before the tank arrives.
Your contents insurance. Water damage from an aquarium is treated differently by different insurers — some cover it, some exclude it, some cover it only up to a stated volume. If the tank is substantial, it is worth a call.
What is underneath you. A leak into your own floor is an expense. A leak into someone else’s ceiling is an expense and a relationship, and it changes where in the flat you would rather put the tank.
None of this is a reason not to keep fish. It is the same argument as the rest of the page: the decisions are cheap to make before the tank is full and impossible afterwards.
Where a tank should not go
| Place | Why not |
|---|---|
| Direct sunlight | Algae, and a temperature swing every sunny afternoon |
| Beside or above a radiator | The same swing, from the other direction |
| In front of a window | Backlit fish, worse algae, cold in winter |
| A draughty hallway or beside an external door | Fast temperature change is what goldfish handle worst |
| A conservatory | Extremes at both ends of the year |
| On or beside a speaker | Continuous vibration is a continuous stressor |
| A busy corridor | Knocks, and fish startled a dozen times a day |
| Against a wall with no access behind | You will regret it at the first filter service |
| Above a stairwell, or over a downstairs flat | Consider the leak, not just the fish |
| Far from a tap and a drain | Every water change for the next decade |
The temperature points are not fussiness. The temperature guide sets out why goldfish tolerate a wide range and a fast change badly, and a tank in a sunny window or beside a radiator gets a fast change most days. Sunlight also feeds algae far more effectively than any aquarium light, which is a maintenance cost you sign up to permanently.
Access and clearance
Plan the space around the tank, not just the space it occupies.
Above: enough room to lift the lid, get a net in, and run a siphon. For a tall tank on a tall stand, check you can actually reach the bottom with your arm.
Behind: most external filters and hang-on-back units need 5 to 15 cm. Check the specific model before the tank is against the wall.
In front: room to kneel with a bucket.
Beneath, if the stand is a cabinet: the external filter, the buckets, the test kit and the food, which is worth planning rather than discovering.
A socket within reach, without an extension lead running under the tank where a leak would find it.
Electrical safety
Brief and worth stating plainly, because water and mains electricity are in the same room permanently.
A drip loop on every cable. Arrange each cable so it dips below the socket before rising to it. Water running down a cable then falls from the low point instead of into the plug. It costs nothing and it is one of the main protections.
An RCD on the circuit. Standard in most modern consumer units; worth confirming.
Nothing on the floor under the tank that a leak would reach — extension leads, plug strips, chargers.
Unplug before putting your hands in the water for anything more than a quick net, and dry your hands before touching a plug on the way back.
Keep the sockets above the water line, not on the skirting beneath the tank. A drip loop protects the cable; it does not help a socket sited where a seam failure will reach it.
Watch the total load on one strip. A filter, heater, air pump, light and sometimes a UV unit end up on a single extension lead behind a cabinet, in a warm and humid spot. Check the strip is rated for the sum, and replace it if it is old — it is the cheapest component in the room and the one carrying everything.
Heaters have their own rule: never run one in air. Unplug it before a water change drops the level past it, and after the level comes back up leave it unplugged for as long as its instructions say before switching it on, so the glass is not hot in cold water. Most modern heaters have a thermal cutout; the cheap ones crack.
Filling, and never moving it full
Position the stand, level it, put the tank on, level it again, then fill. In that order.
A tank is never moved with anything in it. Not water, not substrate, not “just a bit”. A glass aquarium is designed to be supported evenly across its base, and lifting or dragging a loaded one flexes the base and twists the frame. The result is a cracked bottom pane or a split seam, sometimes immediately and sometimes six weeks later. If the tank has to move even 30 cm, it comes apart first — and the handling and moving guide covers moving the fish and the filter media while you do it.
Fill slowly onto a plate or your hand to avoid digging a crater in the substrate, and check the level again at a quarter full.
Ponds
A pond dug into the ground puts its weight onto the ground, which will take it. The arithmetic only reappears in three cases:
- Raised ponds, where the retaining structure carries the water. A modest 1,000 litre raised pond is a tonne pressing outward on its walls, and the outward pressure at the base is what fails.
- Balconies and roof terraces, where the same question as an upstairs tank applies with a much larger number attached.
- Decking, which is built for people walking on it.
All three are questions for whoever built the structure. The pond guide covers the rest of the siting decision — sunlight, leaf fall, depth and predators — which are the considerations for a pond at ground level.
Before you buy the tank
Five minutes with a tape measure saves the common outcome, which is a tank that fits the room and not the practicalities:
- Measure the space, including behind and above.
- Work out the loaded weight from the table, and decide where in the room it goes.
- Find the joists, if the floor is suspended.
- Check the route from the door to that spot, empty tank in hand.
- Check the distance to the nearest tap, and be realistic about it.
Then buy the tank, and buy it once. The choosing a tank guide covers what to look for in the tank itself, and the setup guide covers the order everything goes in.
Frequently asked questions
How much does a goldfish tank actually weigh?
Roughly 1.4 kilograms for every litre of nominal volume once it is running, which surprises most people. Water alone is 1 kg per litre; the glass adds 15 to 20 percent of that on a typical tank, substrate adds more, and rock, equipment and the stand add the rest. A 100 litre tank is therefore around 140 kg all in, a 200 litre around 270 kg, and a 350 litre around 470 kg — all of it standing on a footprint no larger than a desk.
Can I put a fish tank upstairs?
Usually yes, with thought about where. A small tank on a sound floor is rarely a concern. As a rule of thumb, from about 100 litres upward on a suspended timber floor, placement starts to matter: run the tank across the joists rather than along one, put it close to a load-bearing wall rather than in the middle of a span, and avoid the centre of a large room. Above about 250 litres upstairs, the answer is to ask a builder or structural engineer rather than a website.
Can I use a sideboard or a chest of drawers as a stand?
Usually no, and it is a common and expensive mistake. Ordinary furniture is designed for a light, distributed load — books, crockery, a television — not for a quarter of a tonne concentrated on its top panel. A purpose-built aquarium stand transfers the weight down through a frame to the floor at its corners and perimeter, which is a different structure doing a different job. If you do reuse furniture, it needs a solid top, a frame beneath it rather than four thin legs, and a footprint at least as large as the tank base.
Does the tank need to be perfectly level?
Yes, and it matters more than it looks. A tank that sits even slightly out of level puts uneven stress on the silicone seams and unsupported pressure on the base, which is how tanks split along a seam months after they were filled. Level the stand in both directions with a spirit level before the tank goes on, shim under the stand rather than under the tank, and check again once the tank is a quarter full — floors deflect under load, and the reading before filling is not the final one.
Where should I not put a goldfish tank?
In direct sunlight, beside or above a radiator, in a draught by an external door, in a conservatory, or anywhere you cannot reach the back. Sunlight drives algae and swings the temperature; heat sources do the same from the other direction; draughts and conservatories produce the fast temperature changes goldfish handle worst. Beyond the fish, think about the leak: a tank above a stairwell, over a downstairs flat, or on a floor you would hate to replace is a gamble that occasionally does not pay.
What is a drip loop and do I need one?
A drip loop is a deliberate dip in every cable between the equipment and the socket, so that water running down the cable falls off at the lowest point instead of continuing into the plug. You need one on every cable that enters the tank — filter, heater, light, air pump — and it costs nothing but arranging the cable properly. A drip loop and an RCD on the circuit are two of the main protections for an aquarium, alongside unplugging before you put a hand in the water.
Should I fill the tank before or after putting it in place?
Always after, and never move a tank with anything in it. A glass aquarium is designed to be supported evenly across its base; lifting or dragging one with water or substrate inside flexes the base and twists the frame, which cracks the bottom pane or splits a seam. Position it empty, level it, then fill it where it stands — and if it ever has to move again, it comes apart first.
Do I need to worry about the weight of a pond?
Not in the same way. A pond dug into the ground carries its weight on the ground itself, which will take it. The exceptions are raised ponds and anything on a roof terrace, balcony or decked area, where the same arithmetic applies with considerably more force — a modest 1,000 litre pond is a tonne of water, and that is a question for whoever built the structure it is standing on.