04 Aquarium
Building a goldfish pond
The pond guide sets out what a goldfish pond needs. This is how to make one — starting with the two decisions that cannot be changed afterwards, which are where it goes and how deep it is.
The pond guide sets out what a goldfish pond needs to be: how deep, how big, how stocked, and what happens to it across a year. This is the page about making one.
Two decisions cannot be revisited once the pond exists: where it is and how deep it goes. Everything else — filtration, planting, edging, stocking — can be changed later at moderate cost. Those two cannot, so they come first and they deserve more thought than the rest combined.
Siting
Sun, but not all day. Four to six hours is a reasonable target. Full sun from morning to evening drives algae and pushes summer temperatures up in a body of water that cannot move away from it. Deep shade suits neither plants nor fish.
Not under deciduous trees. Autumn leaf fall into a pond is a serious load of decomposing organic matter, and roots find liners. If a tree is unavoidable, plan on netting the pond every autumn for its whole life.
Not at the lowest point of the garden. Runoff finds the low ground, and garden runoff carries fertiliser, pesticide, soil and whatever was on the path. A pond slightly raised relative to its surroundings, or with the ground falling away from it, is far easier to keep clean.
Within reach of power and water. A pump needs a socket on a protected outdoor circuit, and topping up needs a hose that reaches. Both are much cheaper to plan than to retrofit.
Where you will actually see it. A pond you walk past daily is a pond whose problems you notice. One at the end of the garden is one you visit when something has already gone wrong.
Size and depth
Depth: deeper than the ice your winters make — about 60 cm is commonly advised for mild winters, and 80 to 90 cm or more where hard frosts persist.
The reasoning is thermal and it is covered from the feeding side on the seasonal page: a deep pond holds a large volume that changes temperature slowly, and because water is densest at 4 °C, the bottom layer stays unfrozen under the ice as long as the ice does not grow down to it, while a shallow pond tracks the weather. Depth and steep sides also reduce the places a heron can wade, but do not remove the risk.
Volume: 1,000 litres is a sensible floor for a small group, and several thousand is far easier to keep stable.
Surface area matters as much as volume, because gas exchange happens at the surface. A wide, shallower pond and a narrow, deeper one of the same volume behave very differently — the first breathes better, the second overwinters better. A pond with both is the goal: a broad surface and a deep zone within it.
Build it bigger than you think you need. A pond is enormously easier to enlarge before it exists. The extra cost at the digging stage is a few hours and a slightly larger liner; afterwards it is the whole project again.
A goldfish pond in cross-section
Comet and wakin: Yatokin, CC BY-SA 4.0
- Deep enough not to freeze solid: 60–90 cm or more where winters freeze, deeper in harder climates. Keep a hole in the ice for gas exchange.
- A planted shelf for marginal plants that shade the water and take up nitrate.
- Pump and filter sized for the pond’s volume, with the intake protected.
- Waterfall or return that keeps the surface moving for oxygen and gas exchange.
- Netting or cover against herons, cats and raccoons, and to keep autumn leaves out.
- Floating plants for shade and cover in summer.
Liner options
| Type | Life | Verdict |
|---|---|---|
| EPDM / butyl rubber | EPDM about 25–30 years quoted, butyl about 15–20 | The default for most ponds. Conforms to any shape, stays flexible in cold, tolerates sunlight |
| PVC liner | About 10–15 years quoted | Cheaper, less forgiving, can become brittle |
| Preformed rigid shell | Long | Easy to install, and often too shallow and too small for goldfish |
| Concrete or rendered block | Permanent | Expensive, and must be sealed — see below |
| Fibreglass | Long | Durable and costly; usually a custom job |
These are suppliers’ figures and they disagree: one quotes 25 to 30 years or more for EPDM, another a decade at most for EPDM and PVC.
EPDM with a geotextile underlay is the right answer for most people building once.
The concrete warning is not a formality. Uncured or unsealed concrete leaches free lime into the water, raising pH to levels that kill fish. A concrete pond must be fully cured and then sealed with a pond-safe coating, or treated and flushed repeatedly until the water chemistry is stable. Test before any fish goes anywhere near it.
Sizing the liner
Length = pond length + (2 × maximum depth) + overlap at each end. Same arithmetic for the width.
For a pond 3 m × 2 m and 0.9 m deep, with 30 cm of overlap:
- Length: 3 + 1.8 + 0.6 = 5.4 m
- Width: 2 + 1.8 + 0.6 = 4.4 m
Round up. A liner 20 cm short is a liner bought twice.
Digging and shaping
Mark it out first with a hose or rope and live with the shape for a day before cutting turf.
A planting shelf, but not all the way round. A shelf 20 to 30 cm deep along part of the edge holds marginal plants. A shelf that runs the whole perimeter is a continuous wading platform for a heron, which is the wading access that depth and steep sides are meant to reduce. Steep sides elsewhere, and a defined deep zone.
A flat bottom in the deep zone, so the fish have somewhere level to sit out winter and so debris collects predictably rather than in a dozen pockets.
Remove everything sharp. Stones, roots, glass, anything in the soil. Then run a hand over the whole surface. A liner is tough and it is not proof against a flint under 900 mm of water pressure.
Underlay, liner and filling
Underlay first. A geotextile pond underlay is the proper material; sand works on a smooth base; old carpet is the traditional substitute and works if it is clean. Cover the whole excavation including the sides.
Lay the liner on a warm day. Rubber is supple in the sun and stiff in the cold, and a supple liner settles into corners in a way a cold one fights.
Pleat, do not cut. Fold the excess into neat pleats at the corners and tuck them under the edging. A cut liner is a liner with a seam.
Fill slowly, letting the liner settle and pull into shape as the weight comes on. Walk the edges as it fills and ease pleats where they are pulling.
Trim last, once the pond is nearly full and the liner has stopped moving, leaving a generous margin to go under the edging.
Edging, overflow and an exit for wildlife
Edging hides the liner and stops runoff. Slabs, stone, turf over the liner edge — whatever suits the garden. The key point is that the liner should turn up at the edge and terminate above the water line, so ground water cannot run in over the top of it.
An overflow is not optional. Heavy rain fills a pond, and without a controlled route out the excess leaves over the lowest point of the edge, taking fish with it in a serious storm. A pipe through the edging to a soakaway or a border, or a deliberately lower section of edge leading somewhere harmless, is enough.
A way out for anything that falls in. Frogs, hedgehogs and small mammals get into ponds and drown against vertical sides. A shallow sloping section, a partly submerged log, or a ramp of stones at one point solves it. This costs nothing at the building stage and is the difference between a pond that supports garden wildlife and one that quietly kills it.
Pump, filter and plumbing
For goldfish a pond needs filtration. A wildlife pond with no fish reaches its own balance; a pond stocked with animals producing as much waste as goldfish does not — the argument is the same one the filtration guide makes indoors.
The usual arrangement: a submersible pump in the deep zone, feeding a filter box sitting above the water level at the pond edge, which returns by gravity — often through a waterfall or a spill, which aerates as it goes.
Sizing the pump: turn the pond volume over roughly every one to two hours. A 3,000 litre pond therefore wants a pump moving somewhere around 1,500 to 3,000 litres an hour at the height it has to lift, which is not the same as the figure on the box — pump output falls as head height rises, and the specification sheet will give a curve. The pond pump buying guide puts real numbers on that curve, and covers the specification that matters most for goldfish — how large a solid the pump will pass through to the filter.
A UV clarifier in line deals with the single-celled algae that produce green water, and the UV buying guide covers sizing one. It is aimed at suspended algae rather than string algae, and it is not a filter; it is a clarifier, and the algae page covers the distinction.
Winter arrangements matter at the design stage: the pump should be liftable to a shallower position for the winter, which pond suppliers advise so that it does not draw the slightly warmer deep water up towards a freezing surface. The power cuts page covers what happens when it stops.
Filling, treating and cycling
A new pond is an uncycled system, exactly like a new aquarium.
- Fill it, and check the level and edges as it goes.
- Treat the whole volume for chlorine and chloramine. The source water page covers what is in your supply and why standing no longer removes it.
- Plant it. Plants seed the system with bacteria and start consuming nutrients immediately.
- Run the filter and leave it running.
- Wait weeks, not days. The cycling guide applies without modification; a pond filled last weekend is not ready for fish.
- Then stock slowly, a few fish at a time, testing as you go.
Raised and semi-raised ponds
Worth considering rather than treating as a compromise, and genuinely different to build.
Why people choose one: ground that cannot be dug — solid rock, a high water table, services, a paved area — or a garden where a raised pond is easier to see into, easier to maintain without kneeling, and easier to make safe around children.
The engineering is different. A dug pond’s walls are held by the surrounding earth. A raised pond’s walls have to hold the water in by themselves, and water pushes outward hardest at the bottom. A retaining structure that would be ample for a raised bed of soil is not necessarily ample for the same volume of water, and this is the part people underestimate — the weight arithmetic that applies to an indoor tank applies here with a much larger number attached.
Semi-raised is often the best of both. Dig the deep zone into the ground, build the upper part above it. The earth carries the pressure where it is greatest, the raised portion gives the viewing height and the edge to sit on, and the depth needed for overwintering is achieved without a wall holding back a tonne of water.
On a balcony, terrace or decking, the question is not the pond at all but what is underneath it, and that is a conversation with whoever built the structure.
Decide these before you dig
A short list of things that are straightforward at the excavation stage and impossible afterwards.
A bottom drain, if you want one. Standard on serious koi ponds: a drain at the lowest point, plumbed to an external filter, which removes settled waste continuously instead of leaving it on the floor. It cannot be retrofitted through a liner without draining and rebuilding. Most goldfish ponds do not need one. Deciding you want one later is expensive.
A skimmer, which pulls floating debris off the surface before it sinks and rots. Same argument: much easier to build in.
Where the pipework runs, and a route for cable and hose that does not cross a path or a lawn you will mow.
How you will net it in autumn. A pond with an awkward outline is a pond you cannot cover easily, and leaf netting is an annual job for its whole life.
The cover or fence, if children have access. Designing a shape a grid will sit on is trivial at the marking-out stage.
When to build
Spring or early summer, for a reason that has nothing to do with the weather being pleasant.
A new pond has to cycle, and cycling runs on bacterial activity that follows temperature — the same relationship the temperature guide describes for the fish. A pond filled in April establishes over warm weeks, plants root and grow, algae blooms and then settles as the planting takes over, and the first fish arrive with a whole season ahead of them to feed and put on condition.
A pond filled in September does none of that. It cycles slowly or not at all, the planting does not establish, and any fish added go into winter in a new system without the autumn conditioning the seasonal page argues is what carries them through. It is the same argument as not buying fish in autumn, applied to the whole pond.
If the digging happens in autumn anyway, fill it, plant what will take, run the filter through the winter, and stock in spring.
What it costs
No figures here, for the reasons the costs page sets out, but the shape is worth knowing: excavation, liner and the pump-and-filter set dominate, and everything else — underlay, edging, plants, treatments — is small beside them. Labour is the variable, since a pond is either a weekend with a spade or a day with a machine and an operator.
The running cost is lower than an indoor tank per fish: no heating at all, a pump as the only continuous draw, and no feeding for part of the year.
Planting
Briefly, since the plants guide covers the species question.
Marginals on the shelf — irises, rushes, marsh marigold — which use nutrients and soften the edge.
Oxygenators in the deep zone, which do real work on water quality.
Floating cover — water lilies or floating plants over perhaps a third to a half of the surface, which shades the water, suppresses algae and hides fish from birds.
And the goldfish caveat: they eat soft-leaved plants and dig at roots. Plants in baskets with stone over the compost survive; delicate species planted directly into a pond with adult comets in it do not.
Safety and permissions
Children. A pond of any depth is a drowning hazard for a small child, and shallow water is not safe water. If children have access, the answer is a rigid mesh or grid at or just below the surface, or a fence with a self-closing gate. Decide this before digging, because retrofitting a cover to an irregular shape is much harder than designing for one.
Local rules. Some jurisdictions regulate pond depth, fencing or both. Rented property may prohibit excavation entirely. Worth five minutes before a day with a spade.
Electrical work outdoors belongs to someone qualified where you are not. An outdoor socket for a pond pump should be on a protected circuit.
What goes wrong
| Mistake | Consequence |
|---|---|
| Too shallow for the winter | Water that freezes through; easier wading for herons |
| Not level | A band of liner visible for the pond’s whole life |
| Shelf all the way round | A wading platform for predators |
| Liner undersized | Bought twice |
| No overflow | The pond empties over the edge in a storm |
| Under-filtered | Green water, high nitrate, unhappy fish |
| Fish added too soon | The pond cycles around them |
| Built under a tree | Leaves every autumn, forever |
| Sharp material left in the excavation | A slow leak that is very hard to find |
Afterwards
Once it is running, the pond stops being a construction project and becomes a seasonal one: what to do across the year, when to feed and when to stop, and the annual autumn work that decides how the fish come through winter.
And when the first fish go in, move them properly — a tub of pond water, acclimated over hours rather than minutes, as the handling guide describes. A pond built carefully deserves a careful introduction.
Frequently asked questions
How deep does a goldfish pond need to be?
The deepest point has to be deeper than the ice your winters make. About 60 cm is commonly advised for mild winters, and 80 to 90 cm or more where hard frosts persist. Depth buys thermal stability: water is densest at 4 °C, so in a pond deep enough for its climate the bottom layer stays unfrozen under the ice, while a shallow one tracks the weather and can freeze through. Depth and steep sides also reduce the places a heron can wade, though they do not remove the risk. If you are building once, build deeper than the minimum — it is one of the two decisions that cannot be revisited.
What size liner do I need?
Length equals the pond's length plus twice its maximum depth plus an overlap at each end, and the same arithmetic for the width. For a pond 3 m by 2 m and 0.9 m deep, with 30 cm of overlap: 3 + 1.8 + 0.6 = 5.4 m by 2 + 1.8 + 0.6 = 4.4 m. Round up rather than down, because a liner 20 cm short is a liner you buy twice, and the offcut costs almost nothing by comparison.
Which liner material is best?
EPDM rubber for most people. It conforms to any shape, stays flexible in cold weather and tolerates ultraviolet, and suppliers quote about 25 to 30 years for it, though they do not all agree. PVC is cheaper and shorter-lived, with about 10 to 15 years quoted, and can go brittle. Preformed rigid shells are easy but often too shallow and too small for goldfish. Concrete is permanent and expensive, and it must be properly sealed — uncured concrete leaches free lime that raises pH to levels that will kill fish.
Where should a pond not go?
Directly under deciduous trees, because of leaf fall and roots. At the lowest point of a garden, where runoff carrying fertiliser, pesticide and soil will find it. In full sun all day, which drives algae and heat. In deep shade, which suits nothing. And nowhere until you have established what is buried where you intend to dig — services under a lawn are the one pond-building mistake that can hurt you rather than the fish.
Do I need a filter on a goldfish pond?
For goldfish, yes. A wildlife pond with no fish balances itself; a pond stocked with animals that produce as much waste as goldfish does not. The usual arrangement is a submersible pump feeding a filter that sits above the water level and returns by gravity, often with a UV clarifier in line to keep green water down. Size the pump to turn the pond's volume over roughly every one to two hours.
How long before I can add fish?
Weeks, not days. A new pond has no bacterial colony, exactly like a new aquarium, so it has to cycle before it can process waste. Fill it, treat the whole volume for chlorine and chloramine, plant it, run the filter, and let it establish — planting helps and patience does the rest. Adding fish to a pond filled last weekend is the same mistake as adding them to a tank set up on Saturday.
Is a pond safe with small children around?
Not inherently, and this matters more than anything else on the page. A pond of any depth is a drowning hazard for a small child, and shallow water is not safe water. If children have access to the garden, a rigid mesh or grid at or just below the surface, or a fence with a self-closing gate, is the answer — and local rules may require one. Decide this before you dig, because retrofitting a cover to an awkward shape is considerably harder than planning for it.
Do I need an overflow?
Yes, and it is routinely forgotten. A pond fills in heavy rain, and without somewhere controlled for the excess to go it will find its own route — over the lowest point of the edge, taking fish with it in a bad enough storm. A simple overflow pipe or a deliberately lower section of edging leading to a soakaway or a border solves it, and it costs almost nothing at the building stage.