What kind of pond for goldfish? Size, depth, and filtration for them to live twenty years

Goldfish have an unfair reputation. They are imagined as tiny and fragile, doomed to spend a few months swimming in circles in a bowl. In reality, in a well-designed pond, they are robust fish that often exceed 20 cm and can live for around twenty years. The difference between these two fates depends almost entirely on the pond: its volume, depth and water quality. Here is how to design it correctly from the outset.

What the goldfish really needs

The goldfish is a domesticated form of a wild cyprinid from East Asia, selectively bred in China for more than a thousand years. It has retained the hardiness of its ancestors: it tolerates cold water in winter, warm water in summer and significant fluctuations, provided these changes remain gradual. Its real needs are simple: space to swim, clean, well-oxygenated water, and shelter from the cold and predators.

Be careful not to confuse the different body shapes. Common goldfish, comets and shubunkins, with their elongated bodies, are perfect for outdoor ponds year-round. Fancy varieties with round bodies, such as fantails, orandas and ryukins, are slower and more sensitive to the cold: they are better suited to an aquarium or a very sheltered pond.

Dimensions to plan for

Depth is the criterion most often underestimated. In winter, fish gather in the deepest area, where the water remains around 4 °C. A 40- or 50-cm pond can freeze almost to the bottom during a severe winter, and a shallow pond heats up dangerously in summer. A deep zone, even if limited to part of the pond, stabilizes the temperature all year round.

How many goldfish should you put in?

Start small. A population of three to five fish in a 1,000-litre pond is reasonable. This may seem like few, but goldfish grow and reproduce easily: a pair can produce dozens of young in its first summer if plants provide hiding places. An overcrowded pond always results in cloudy water, algae and disease. In case of overpopulation, give the fish to a garden centre or another enthusiast, but never release them into a pond or waterway: they compete with native species and degrade water quality.

Liner or preformed pond?

Preformed ponds are practical and quick to install, but many common models are shallow. Always check the maximum depth before buying: for fish that stay outdoors in winter, a model that does not reach 80 cm should be reserved for plants or a decorative pond. EPDM liner laid over protective felt lets you freely choose the shape, surface area and, above all, depth, at a much lower cost than a large preformed pond. This is the solution we recommend in most cases.

Filtration and oxygen

A small planted pond with two or three fish can sometimes function without a filter. As soon as the population increases, or if you want to see your fish in clear water, a filter becomes essential. Choose one rated for a volume greater than yours: manufacturers' specifications often correspond to a sparsely populated pond. A biological filter with an integrated UV lamp also deals with suspended green algae. In summer, an aerator or waterfall supplements the oxygen supply, especially at night and during stormy weather.

Plants, allies of goldfish

Submerged plants oxygenate the water and consume the nutrients that algae feed on. Water lilies shade the surface and provide shelter from herons. Marginal plants filter runoff. Finally, young goldfish find refuge in dense vegetation. A well-planted goldfish pond requires significantly less maintenance than a bare pond.

Feeding without polluting

In a planted pond, goldfish find some of their food on their own: larvae, algae, and small invertebrates. Give them a quality food in small quantities, consumed within a few minutes, once or twice a day in summer. Reduce feeding as the water cools and stop below approximately 8 °C: their digestion then slows down.

Also read

For more information: How long do pond goldfish live?.

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Frequently asked questions

— What is the minimum size for a goldfish pond?

Allow at least 1,000 liters for a few fish, with an 80 cm-deep zone if the pond remains inhabited in winter.

— Can goldfish overwinter outdoors?

Yes, common goldfish, comets, and shubunkins overwinter very well in a sufficiently deep pond. Keep a hole in the ice for gas exchange.

— Is a pond without a filter suitable?

Only if it is large, heavily planted, and sparsely populated. In most cases, a filter prevents green water and protects the fish's health.

— Can goldfish be kept with koi?

Yes, they coexist well. The pond must then be sized for koi, which require much more volume.

In summary

• At least 1,000 liters and an 80 cm-deep zone to overwinter.
• A modest initial population: goldfish reproduce quickly.
• An oversized filter, with plants covering one third of the surface.
• EPDM liner is the most flexible solution for achieving the right depth.

Last updated: September 28, 2026.