Koi

People usually start searching for koi information when they’re making quick decisions: can koi live in my backyard pond, what water conditions do they actually need, and what goes wrong when things slip. A koi pond looks calm on the surface, but the biology underneath is unforgiving—poor water quality and sudden temperature swings can turn into disease, losses, and a lot of work very quickly.

Koi are also domesticated forms of common carp, and in Australia that matters. Depending on your state and local rules, keeping or moving koi can be restricted because carp are an introduced pest species, and releases into waterways are a serious environmental risk.^{1, 2, 3}

Quick facts (at a glance)

• Scientific name: Cyprinus carpio (koi are ornamental varieties of common carp).¹
• Adult size: commonly 45–75 cm in ponds; large carp can exceed 1 m in some conditions.¹
• Longevity: carp can be long-lived (30+ years recorded; longer overseas reports exist).⁴
• Temperature window for spawning (carp): often around 17–25°C.⁴
• Diet: omnivorous; in ponds they’re typically fed a formulated pellet diet, with natural grazing on invertebrates and plant material as available.⁵
• Behaviour: shoaling fish; they cope better with stable conditions and adequate space than with frequent handling or constant change.
• Cost: varies widely with size, pattern and breeder line; the biggest ongoing cost is usually filtration, electricity, testing and feed rather than the fish itself.

What is a koi (and why Australia treats them differently)

Koi—often called “Japanese carp”—are domesticated ornamental strains of common carp (Cyprinus carpio). They’re the same species as wild carp, just selectively bred for colour, scale types and patterning.¹

In Australia, common carp are an introduced pest that can dominate waterways and damage aquatic habitats by stirring sediments and uprooting plants. That’s why rules around keeping, selling or moving carp (including koi) can be strict, and why “never release pond fish” isn’t just good manners—it’s essential biosecurity.^{2, 3}

Before you buy: check local rules

Carp regulations vary by state. As examples, government agencies in NSW and Queensland classify carp as pest/noxious/restricted, and Victoria has treated koi carp as a noxious aquatic species with restrictions on keeping and movement.^{6, 7, 2}

History and origin (a short, accurate version)

Ornamental koi were developed in Japan from common carp through selective breeding, with colour varieties becoming culturally significant over time. International trade later spread koi keeping well beyond Japan, where they became a familiar feature of garden ponds and public water features.¹

Common koi varieties (what the names actually mean)

Koi varieties are mostly judged by base colour, pattern placement, and the presence or absence of black (sumi) and metallic sheen. A few you’ll see most often:

• Kohaku: white base with red patterning.
• Taishō Sanshoku (Sanke): white base with red and black markings.
• Shōwa Sanshoku (Showa): black base with red and white markings.
• Utsurimono: black-based koi with a second colour (often white, red or yellow).
• Gin Rin: “sparkling” scales; can appear across multiple colour varieties.

Names are useful shorthand, but health and conformation matter more than the label. A koi with clean fins, steady appetite, and unfrayed skin will outlast a “rare” fish living in unstable water.

Backyard pond care: the conditions koi actually need

Koi survive a wide range of conditions, but “survive” is not the same as “thrive”. Their best days come with consistency: stable water quality, reliable filtration, and enough volume that the pond doesn’t swing wildly between morning and afternoon.

Pond size and depth (realistic expectations)

There isn’t a single perfect number because stocking depends on filtration, feeding rate and plant load, but koi are large-bodied fish that produce a lot of waste. In practice, bigger and deeper ponds are easier to keep stable, especially through heatwaves and cold snaps. If you’re planning a small pond, fewer koi is almost always the safer choice.

Filtration and oxygen

Most koi losses trace back to water quality. Effective filtration needs two jobs covered:

• Mechanical filtration: removing solids before they break down.
• Biological filtration: supporting beneficial bacteria that process nitrogenous waste.

Good surface movement and aeration matter because low dissolved oxygen can follow warm weather, heavy feeding, algae die-off, or a sudden load of decaying organic matter.⁵

Feeding: less romance, more observation

A quality pellet formulated for koi is the backbone of most pond diets. Feed to appetite, not to the label on the tub. One practical rule borrowed from pond management is to only offer what fish can consume within about 10–15 minutes, and adjust down if water quality slips or fish go off their food.⁵

Cold water slows digestion. In cooler months, many keepers reduce feeding sharply or stop altogether when fish are no longer actively feeding, rather than letting uneaten food rot into the filter and silt.

Breeding and genetics (what’s true, and what’s oversold)

Carp (including koi) reproduce by external fertilisation, with adhesive eggs typically laid on vegetation in shallow water during warmer periods. In the wild, carp can be highly prolific under suitable conditions.⁴

Selective breeding is how koi varieties are made: breeders choose parent fish for pattern, colour stability, scale type and body shape. For backyard pond keepers, the practical takeaway is simple—unplanned breeding can quickly increase stocking pressure and strain water quality, even if only a small fraction of fry survive.

Koi health issues: what usually causes trouble

Disease in koi ponds is rarely a random lightning strike. It more often follows a chain: stressor → water quality decline → parasite or bacterial bloom → secondary infections.

Common triggers

• Rapid temperature changes (including shallow ponds heating fast).
• Overstocking and heavy feeding without matched filtration.
• New fish introductions without quarantine.
• Poor oxygenation during warm weather or after algae die-off.⁵

Koi herpesvirus disease (KHV / CyHV-3)

Koi herpesvirus disease is internationally recognised and listed by the World Organisation for Animal Health (WOAH). It affects common carp and ornamental koi, and it’s one reason responsible keepers take quarantine seriously and avoid sourcing fish of unknown health status.⁸

In Australia, cyprinid herpesvirus-3 has been researched as a potential carp biocontrol agent, with work focusing on host specificity and broader ecological considerations. Government and research bodies have also stated that the virus has not been released in Australia.^{9, 10}

Koi appreciation and judging (a gentle primer)

Koi shows and judging tend to focus on health and structure first—body shape, skin quality, fin condition—then pattern balance and colour clarity. The best fish look effortless in the water: a smooth body line, steady swimming, and crisp pattern edges that hold as the fish grows.

Practical red flags (pond and fish)

• Fish gasping at the surface or clustering near waterfalls: often oxygen or water quality trouble.
• Sudden loss of appetite across multiple fish: treat it as a water-quality alarm until proven otherwise.⁵
• Flashing (rubbing), clamped fins, or repeated jumping: can indicate irritation from parasites or poor water.
• New fish added straight into the pond: one of the quickest ways to introduce disease.

References

1. NSW Department of Primary Industries — General information about carp (includes koi as ornamental varieties of common carp)
2. Victorian Fisheries Authority — Reminder that koi carp are noxious
3. Australian Government Department of Agriculture, Fisheries and Forestry — Carp Biological Control Program
4. Murray–Darling Basin Authority — Fish fact sheet: Carp (Cyprinus carpio)
5. University of Florida IFAS Extension (EDIS) — Managing Florida Ponds for Fishing (feeding rate guidance and oxygen risk from overfeeding)
6. NSW Department of Primary Industries — Carp (Cyprinus carpio) species information
7. Business Queensland — Carp (restricted noxious fish under the Biosecurity Act 2014)
8. World Organisation for Animal Health (WOAH) — Koi herpesvirus disease
9. McColl et al. (2017) — Cyprinid herpesvirus 3 as a potential biological control agent for carp in Australia: susceptibility of non-target species (PubMed)
10. Fisheries Research and Development Corporation (FRDC) — National Carp Control Plan research (notes the carp virus has not been released in Australia)