Koi (Cyprinus rubrofuscus)

What is a koi?

Koi (Cyprinus rubrofuscus), known in Japan as Nishikigoi — literally “brocaded carp” — are large ornamental carp selectively bred over centuries for spectacular colour and pattern. They are among the most culturally significant fish in the world, appearing in Japanese gardens for hundreds of years as symbols of perseverance, good fortune and longevity.

Adults reach up to 90 cm (35 in) and weigh several kilograms, which places koi firmly in the category of pond fish, not aquarium fish. Their peaceable temperament and sociable nature make them excellent pond inhabitants — they will eat from your hand once settled — but their sheer scale and waste output rule out all but the largest indoor setups. Hundreds of named varieties exist, grouped into classes: kohaku (red-on-white), sanke (red, white and black), showa (black with red and white), and many more. If you are buying your first koi, expect to fall down a very enjoyable rabbit hole of pattern classification.

Where do koi come from?

Koi are domesticated descendants of the common carp (Cyprinus carpio), originating in East Asia — likely first selectively bred for colour in Japan during the early 19th century in Niigata Prefecture. Wild common carp range across much of Europe and Asia, living in slow rivers, lakes and floodplains with rich, silty bottoms and abundant aquatic vegetation.

That ancestral origin explains koi’s hardiness: common carp tolerate a wide range of water temperatures, pH and conditions. Today’s ornamental koi are kept worldwide in outdoor ponds — from Japan and China to the UK, North America, Australia and beyond — adapting readily to local climates provided extreme cold or heat is managed.

What size tank does a koi need?

Koi need a pond, not a tank. The frontmatter minimum of 3,800 L (approximately 1,000 US gal) reflects a serious starter pond for a small group; experienced keepers recommend considerably more. A practical rule of thumb is at least 1,000–1,500 L of water per adult fish, with surface area being at least as important as volume — koi are active swimmers that cruise all water levels throughout the day.

Pond depth matters too: a minimum of 1.2–1.5 m (4–5 ft) provides a thermal buffer against summer heat and keeps the bottom from freezing solid in cold winters. Indoor tanks are only appropriate for small juveniles as a temporary measure. If you are planning a permanent koi pond, err on the side of larger — it is far easier to overstock a pond that turns out to be smaller than expected than to rebuild.

What water parameters do koi need?

Koi are forgiving compared to most tropical fish, but that tolerance does not mean water quality is optional — a heavy bioload demands a well-run system.

• Temperature: 4–28 °C (39–82 °F). Optimal active feeding range is 15–25 °C (59–77 °F). They stop eating below 10 °C (50 °F) and their immune system slows.
• pH: 6.5–8.5, with 7.0–8.0 being the comfortable sweet spot.
• Hardness: 9–18 dGH (moderately hard). Koi appreciate some mineral content.
• Ammonia/nitrite: as close to zero as possible. With the waste load koi produce, robust biological filtration — a dedicated pond filter rated well above your pond volume — is non-negotiable.

Test water weekly during warm months and after any major changes. Dissolved oxygen is often overlooked: koi are large, active fish that deplete oxygen quickly, especially in warm summer water. A waterfall, fountain or dedicated aerator keeps levels safe.

What do koi eat?

Koi are omnivores with hearty appetites, and their feeding should be adjusted seasonally to match their metabolism.

• Above 15 °C (59 °F): high-protein pellets (30–40 % protein) form the staple, supplemented with treats like watermelon, lettuce, peas (shelled), earthworms and aquatic insects.
• 10–15 °C (50–59 °F): switch to wheat-germ-based pellets, which are easier to digest at lower temperatures.
• Below 10 °C (50 °F): stop feeding entirely. Koi enter a semi-torpid state and lack the digestive enzymes to process food; uneaten food rots and pollutes the pond.

Feed small amounts two to three times daily in warm weather — only what the fish can consume within five minutes. Overfeeding is one of the primary causes of poor water quality in koi ponds. Keep in mind that koi will also naturally graze on algae, plant matter, and invertebrates in a healthy pond.

Are koi aggressive — and what fish can live with them?

Koi are entirely peaceful toward other fish, which is one reason they are so popular as pond inhabitants. They do not bite or bully tankmates of similar or smaller size in a meaningful way. However, their sheer size and boisterous feeding behaviour can stress or outcompete smaller, delicate fish, so choose companions accordingly.

Good pond companions include large goldfish (common, comet and shubunkin varieties), golden orfe, and tench, which occupy the bottom and act as useful scavengers. Avoid very small fish that koi may inadvertently ingest while feeding, and avoid delicate species that cannot compete at feeding time.

Keep a minimum group of 3 koi — they are shoaling fish by instinct and visibly more relaxed and active in a group. A pond with 3–6 koi of mixed sizes creates natural, engaging social behaviour.

For a detailed breakdown of which species pair well with koi, see Koi tank mates.

How do you tell male and female koi apart?

Sexing juvenile koi (under two years old) is difficult and unreliable. In mature adults, the differences become clearer, particularly in spawning season.

Females grow noticeably broader across the body, especially the belly, when carrying eggs in spring. Viewed from above, a gravid female has a pronounced rounder shape compared to the more streamlined male.

Males develop small, rough white bumps called breeding tubercles on the pectoral fins and along the gill covers during spawning season. Running your fingertip along the pectoral fin of a mature male in spring will feel slightly rough, like fine sandpaper. Outside of spawning season, body shape is the most reliable guide, with females generally being deeper-bodied once mature.

How do koi breed?

Koi spawning is a dramatic, physically demanding event rated Hard for the keeper to manage well. In spring, as water temperatures rise above roughly 18–20 °C (64–68 °F), males chase females persistently and vigorously — sometimes to the point of injuring them against pond edges or walls.

Eggs are scattered among aquatic plants or spawning ropes (dedicated fibrous mats placed in the pond), and a single female can release hundreds of thousands of eggs. The parents provide no care — in fact they will eat the eggs — so breeders remove adults or the spawn immediately if they want to raise fry.

Raising koi fry requires a separate shallow rearing tank or pond, appropriate infusoria and powdered food for the tiny fry, and significant culling: the vast majority of offspring will not display desirable colour or pattern. Producing quality koi is a specialist endeavour that takes years to develop. Casual spawning in a well-planted pond will occasionally succeed, but deliberately breeding for specific pattern varieties is genuinely demanding.

What are common koi diseases?

Koi are hardy but susceptible to several significant diseases, most of which are preventable through good husbandry.

• Koi herpesvirus (KHV): a serious viral disease with no cure; presents as lethargy, gill damage and rapid death. Prevention is everything — quarantine all new fish for at least 4–6 weeks before introducing them to an established pond.
• Ulcer disease (Aeromonas): bacterial ulcers on the body, usually triggered by stress, poor water quality or physical injury. Clean water and a stable environment are the primary prevention.
• White spot / ich (Ichthyophthirius multifiliis): common white spots across the body and fins; again, triggered by stress and temperature fluctuation.
• Parasites (anchor worm, fish lice): external parasites visible to the naked eye on the skin and fins. Regular visual inspection of your fish is the first line of defence.
• Swim bladder issues: buoyancy problems sometimes linked to diet, injury, or infection.

Prevention follows a consistent pattern: maintain excellent water quality, quarantine all new arrivals, avoid sudden temperature swings, and minimise handling stress.

Health note: disease diagnosis in koi is nuanced — symptoms can overlap significantly between conditions. Confirm any identification against a reputable veterinary or fish-health resource, and consult a fish health specialist for suspected KHV or other serious infections before taking action.

How long do koi live?

Koi are exceptional in the fish world for their longevity. A well-kept koi lives 20–50 years — outlasting most other pets and routinely outlasting their first owner. The oldest reliably documented koi, a fish named Hanako, reportedly lived over 200 years, though that figure is not universally accepted by scientists.

In practical terms, a koi purchased as a healthy juvenile today can realistically still be swimming in your grandchildren’s pond. This longevity means the decision to keep koi is not trivial: they require consistent, serious care for decades, and rehoming a large, established koi is genuinely difficult. Give a koi a large, well-filtered pond, attentive seasonal care, and regular health checks, and you may be looking at the same fish for the rest of your life.