Start With Volume, Not With the Fish

Before you think about livestock, work out how much water you actually have. A pond measuring two metres by one and a half metres and sixty centimetres deep holds roughly 1,800 litres, though shelves, planting baskets and a layer of silt will reduce that. Goldfish are sociable, greedy and messy, and they produce a surprising amount of waste. As a working guide, allow at least 100 litres of water per goldfish, and considerably more for larger specimens — a well-fed goldfish can reach 25cm and will keep growing.

Depth matters just as much as surface area. In most of the UK you need a zone at least 60cm deep, and 75cm is safer in the north, on exposed sites or in a pond that freezes hard. That deeper water lets fish move below the frost line in winter and escape the worst of summer heat. Shallow, bowl-shaped ponds look charming but are not fish ponds, whatever the garden centre display suggests.

Filtration, Oxygen and Water Quality

Still water with fish in it becomes stale water surprisingly quickly. Aim for a pump and filter that turn the pond over completely every one to two hours, and choose a filter rated for a pond larger than yours — manufacturers' figures are optimistic. A biofilter full of mature media is what keeps ammonia and nitrite in check, and it takes four to six weeks to establish, so add fish slowly rather than all at once.

• Top up with rainwater where you can; tap water needs a dechlorinator before it goes anywhere near fish.
• Test ammonia, nitrite and pH fortnightly through the growing season. Goldfish are happiest around pH 7–8.
• Keep surface movement going with a pump outlet, small fountain or air stone, especially on still, humid nights when oxygen levels drop.
• Avoid overstocking and overfeeding. Excess food is the single most common cause of green, soupy water and sick fish.

Goldfish, Native Species and the Law

Common goldfish and shubunkins are the obvious choice for a British garden pond: hardy, long-lived and tolerant of cool water. Fancy varieties with bulbous bodies or delicate fins struggle outdoors and are best kept in aquaria.

Native species are a different proposition. Minnows, three-spined sticklebacks, rudd and tench are attractive and interesting, but they need cooler, well-oxygenated, generously sized water and room to shoal — a hundred litres each is a more realistic allowance than for goldfish. Never take fish from a river, canal or wild pond without the landowner's permission and the necessary authorisation, and never move fish between separate water bodies, which spreads disease and invasive species.

One rule is absolute: never release goldfish or any other non-native fish into the wild. It is an offence, and released goldfish do real damage to native wildlife in British waterways. If you can no longer keep your fish, rehome them responsibly.

Planting for Shelter and Shade

Plants do three jobs at once: they shade the water, use up nutrients that would otherwise feed algae, and give fish somewhere to hide. Aim to cover roughly half the surface with floating leaves — water lilies for larger ponds, or frogbit and water soldier in smaller ones.

• Oxygenators such as hornwort and water crowfoot give cover and shelter for fry.
• Marginals like flag iris, marsh marigold and water mint soften the edges and provide shade.
• Submerged bundles of stems or a few clay pipes on the bottom make excellent refuges when fish feel threatened.
• Leave some open water. Heavily planted ponds can starve at night, when plants use oxygen rather than release it.

Herons, Cats and Other Predators

A heron will find a new pond with uncanny speed, and it only takes a few visits to clear it out. Decoys, plastic owls and reflective tape rarely work for long. Practical protection means making the pond awkward and the fish invisible.

• Steep, sheer sides of 30cm or more above the waterline stop herons wading in.
• Netting on a sturdy frame is the most reliable option, though it needs checking and clearing of leaves.
• Overhanging planting, deep water and submerged refuges give fish somewhere to bolt to.
• Remove any nearby perch. Herons hunt from roofs, fences and branches, so a clear approach discourages them.

Otters and mink are a growing problem in some areas, particularly near rivers. If they are local, a securely pegged net or an electric fence around the pond may be the only practical answer.

Feeding and Seasonal Care

Fish in a well-planted pond find a great deal of their own food — insect larvae, daphnia, algae and tadpoles. Feed a good quality sinking or slow-sinking pellet in spring and summer, giving only what they clear in a few minutes. Once water temperatures drop below about 10°C, stop feeding altogether; food rotting in a cold fish's gut does real harm.

In winter, keep a hole open in the ice with a floating pond heater or a pan of hot water rested on the surface. Never smash the ice — the shock wave can injure fish. Come spring, clean out dead foliage, restart the filter, and let the pond wake up gradually before you think about adding anything new.