Koi

The Koi is an ornamental variety of the common carp (Cyprinus carpio), bred for colour and pattern rather than for food. Adults reach a medium size in a well-kept pond and carry markings in white, red, black, orange, yellow, blue, and cream. The fish are bottom and mid-water feeders that root through the substrate and come to the surface at feeding time.

Koi are kept as pond fish rather than aquarium fish. They are social animals that often gather together and learn to recognise the person who feeds them. Many will take food from the hand once they settle in. Their temperament is calm and curious, and they show little aggression toward companions of similar size.

A single Koi can live for decades. Lifespans of 25 to 40 years are common in good conditions, and some individuals in Japan have been recorded living far longer. This long life means a Koi pond is a long-term commitment rather than a seasonal hobby.

Who it suits

In jurisdictions where koi are legally kept, they suit keepers who have outdoor space for a pond and who want a long-lived, low-conflict fish. They are a good fit for people who enjoy watching and feeding fish and who can commit to year-round water care, including winter. They are not suited to small indoor tanks or to keepers who want a short-term pet.

New Zealand readers interested in ornamental pond fish may wish to contact the NZ Aquatic Society for information on species that can be legally kept in New Zealand ponds and waterways.

The common carp was introduced to Japan from mainland Asia and was raised as a food fish in cold rice-growing regions. In the Niigata region on the western coast, farmers kept carp in irrigation ponds to provide protein through long winters.

During the early nineteenth century these farmers began to notice carp with patches of red, white, and pale colour. They set aside and bred the most colourful individuals over many generations. This selective breeding, carried out in the villages around Ojiya and Yamakoshi, produced the first true ornamental Koi.

Koi reached a wider Japanese audience at the 1914 Tokyo exhibition, where coloured carp from Niigata were displayed to the public. Interest grew through the following decades, and named varieties were standardised as the hobby spread to the rest of the country.

From the mid-twentieth century onward, improved air transport allowed live Koi to be shipped abroad. The fish became popular in ornamental ponds across Asia, Europe, and North America, and Japan remains the recognised centre of high-grade Koi breeding.

Koi entered New Zealand waterways and established feral populations in the North Island, particularly in the Waikato region. Their impact on water quality, native fish, and aquatic plants led to their classification as a noxious fish under the Biosecurity Act 1993. The Great South Island Koi Carping programme and MPI biosecurity efforts now focus on tracking and controlling koi populations in New Zealand's freshwater systems.

A Koi has the deep-bodied, slightly flattened shape of a carp, with a broad head, a downturned mouth, and two pairs of barbels at the corners of the mouth. The dorsal fin runs along much of the back, and the body is covered in large overlapping scales, though some varieties are scaleless or carry only a line of mirror scales.

Koi are graded on the clarity and placement of their colour. Judges look for clean white that is free of yellow, deep even red, sharp boundaries between colours, and a pattern that is balanced from head to tail.

Colour and pattern

Koi are grouped into named varieties by their colours and markings. Kohaku is white with red patches, Sanke adds black markings to that base, and Showa is a black fish with red and white. Other types include solid metallic forms and the pale blue-grey Asagi. The skin pattern is the main feature breeders select for, and the same fish can look different as it grows and its colour settles with age.

Keeping koi is illegal in most of New Zealand under the Biosecurity Act 1993. The care information below applies to keepers in jurisdictions where koi are legally permitted.

Most Koi care is water care. A healthy pond depends on adequate volume, strong filtration, and stable conditions, so the bulk of the work is checking equipment and water quality rather than handling the fish.

Grooming

• Skim leaves and debris from the surface and clean filter media on a regular schedule
• Test water for ammonia, nitrite, nitrate, and pH at least weekly
• Carry out partial water changes to keep nitrate low and conditions stable
• Inspect fish at feeding time for sores, frayed fins, or unusual swimming

Exercise

• Provide enough open swimming space; Koi are active and cover the whole pond
• Keep water well oxygenated with a pump, waterfall, or air stone so fish stay active
• Avoid overcrowding, which limits movement and fouls the water

Environment

Koi are an outdoor pond fish in most climates. The pond should be deep enough to stay stable through summer heat and winter cold, with shade to limit algae and a net or cover where herons and cats are a risk. In cold regions the surface must be kept partly open in winter so gases can escape. In New Zealand's South Island, where winters can be particularly severe, a deep pond and an air stone or de-icer are important to prevent the surface from sealing over completely during frosts.

Keeping koi is illegal in most of New Zealand under the Biosecurity Act 1993. The feeding information below applies to keepers in jurisdictions where koi are legally permitted.

Koi are omnivores that in the wild eat insects, plants, algae, and small invertebrates. In a pond they are usually fed a commercial floating pellet formulated for Koi, which keeps food at the surface where uneaten pieces can be seen and removed. Feeding rate depends heavily on water temperature, because a Koi cannot digest food well in cold water.

Treats and supplements

Koi will take occasional treats such as chopped lettuce, peas, watermelon, or orange in warm weather. Colour-enhancing pellets that contain natural carotenoids can deepen red and orange tones. Treats should stay a small part of the diet, with a balanced staple pellet doing most of the work.

A Koi kept in clean, stable water is a hardy animal that can live 25 to 40 years. The great majority of health problems begin with water quality, so high ammonia, low oxygen, or sudden temperature swings often appear before disease does. Parasites and bacterial infections are the next most common causes of illness.

Preventive care

• Keep filtration running and test water on a regular schedule
• Quarantine all new fish before introducing them
• Avoid overstocking and overfeeding
• Reduce feeding as water cools and keep the pond oxygenated year round
• Net the pond against predators that can injure fish

Keeping koi is illegal in most of New Zealand under the Biosecurity Act 1993. The habitat information below applies to keepers in jurisdictions where koi are legally permitted.

Koi are pond fish, not aquarium fish. They grow large, produce a lot of waste, and are active swimmers, so they need far more water than a tank can provide. A dedicated outdoor pond with mechanical and biological filtration is the standard home for a Koi collection. Depth matters as much as surface area, because a deep pond holds a steadier temperature in both summer and winter.

Setup checklist

• Pond liner or formed shell with adequate depth
• Pump and a filter rated above the pond volume
• Aeration through a waterfall, fountain, or air stone
• Net cover or surrounding netting against herons, cats, and falling leaves
• A water test kit and a dechlorinator for tap water top-ups
• Shade or pond plants to limit algae in summer

In New Zealand's South Island, winter water temperatures can drop close to freezing. A pond in those conditions should be at least 1.5 metres deep to give fish a stable thermal zone near the bottom. An air stone or pond heater prevents the surface from sealing over during frosts.

Breeding koi in New Zealand is illegal under the Biosecurity Act 1993, as koi are classified as an unwanted organism. The information below applies to keepers in jurisdictions where koi are legally held.

Koi breed by external spawning rather than live birth. In late spring and early summer, as water reaches roughly 18 to 22°C, females release thousands of small sticky eggs that males fertilise in the water. The eggs cling to plants, roots, or artificial spawning mats. Breeders often move the eggs or the mats to a separate, predator-free tank, because adult Koi readily eat both eggs and newly hatched fry.

Hatching steps

1. Provide spawning mats or fine-leaved plants when water warms in spring
2. After spawning, move the eggs or mats to a clean, gently aerated tank
3. Hold the water at a steady warm temperature and remove white, fungused eggs
4. Once fry hatch, they cling to surfaces and absorb the yolk sac for a few days
5. Begin feeding fine powdered or liquid fry food, then move to larger foods as they grow
6. Sort and thin the fry over the following weeks, since only a small fraction develop good colour