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Koi dog breed
Aquatics

Koi

Koi are selectively bred ornamental carp designed for viewing from above, but their colors do not reduce the needs of a large, muscular, long-lived fish.

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Origin

🇯🇵 Japan is credited with the modern ornamental koi recognized today, which Smithsonian places in early nineteenth-century Japan, developed from domesticated East Asian common-carp lineages with deeper carp-domestication roots in Asia. Sources differ on where the ornamental variety itself was developed: SRAC attributes its development to China, with subsequent Japanese influence.

Overview

Koi are selectively bred ornamental carp designed for viewing from above, but their colors do not reduce the needs of a large, muscular, long-lived fish. They explore, forage, learn feeding routines, interact in loose social groups, disturb soft substrates, and can grow from a hand-sized juvenile into an adult approaching 90 centimeters. Good koi keeping therefore begins below the surface: sufficient water volume, a stable structural pond, contin…

Temperament

socialactive foragerroutine-learninggenerally non-predatory toward similar-sized carpcapable of vigorous feeding competitionsubstrate-disturbing

History

Carp have a long domestication history in Asia, but the modern ornamental fish recognized as Japanese koi emerged much later. Smithsonian places modern Japanese koi in the early nineteenth century, when farmers retained and bred unusually colored carp, eventually producing the many named color and pattern varieties seen today. Genomic research supports koi as a domesticated carp population shaped by human selection for pigmentation and other trai…

Characteristics

Ph Note
Aeration
Filtration
Backup Power
Feeding Type
Housing Type
Release Rule
Hardness Note
Identity Gate
Pond Depth Note
Tank Size Basis
Pond Volume Note
Publication Note
Social Structure
Temperature Note
Temperature Basis
Quarantine Duration
Water Quality Targets
Lifespan Planning Years

Care Essentials

Diet

Use a reputable, complete koi formulation as the staple and match pellet size and texture to the smallest fish that must eat it. Store food sealed, cool, dry, and within the manufacturer's freshness period; old fats and vitamins do not become safe merely because pellets still look normal. Koi are omnivorous foragers, but insects, algae, plants, fruit, vegetables, and treats do not automatically make a balanced diet. Extras should be modest, fish-safe, and counted within the total ration rather than displacing complete food or overwhelming filtration. Feed according to measured water temperature, fish activity, body condition, life stage, and the product's temperature guidance. In the 18–24°C operating range, healthy koi often feed actively, but the correct amount is what the group consumes promptly while maintaining good condition and stable water chemistry. Feed by measured water temperature rather than by the calendar. Stop feeding entirely once water falls below about 10°C, because digestion slows enough that undigested food can rot in the gut. Between roughly 10°C and the 18–24°C optimum band, reduce frequency and quantity and use a readily digested wheatgerm-based food in spring and autumn. Measure the water itself, since a warm afternoon does not warm a deep pond. Resume feeding only when the water is reliably back above about 10°C on a rising trend, not on the first warm day. During heat, warm water holds less oxygen and a heavy meal increases metabolic and waste demand, so oxygen and chemistry can matter more than appetite. Never use feeding to force fish to the surface when they are gasping. Ensure every individual eats: large koi can monopolize floating food, while a weak or visually impaired fish may need a separate presentation. Do not compensate for poor growth with constant high-protein feeding in an undersized pond. Growth depends on genetics, sex, temperature, health, space, and water quality; accelerating one input can increase obesity, deformity, waste, and filter load without improving welfare.

Health

Prevention combines measured water quality, adequate oxygen, low injury risk, nutrition, quarantine, and control of every wet pathway into the pond. Keep ammonia and nitrite as close to nondetectable as the method can show; SRAC's below-0.05 mg/L values are production operating limits, not targets for chronic exposure. Maintain dissolved oxygen at least 5 mg/L and preferably with margin during warm nights and emergencies. Track pH and alkalinity because a stable near-neutral reading is safer than abrupt correction toward a fashionable number. Condition source water, avoid rapid temperature or chemistry changes, and never assume clear water is chemically safe. Quarantine new koi in an independent, cycled, covered system for a minimum of four weeks to two months, extended on a risk basis. That period only tells you something under two conditions that WOAH attaches to it. First, hold the quarantine at a temperature permissive for koi herpesvirus expression: WOAH treats water above 16°C as the range conducive to clinical disease and inspection, and describes the disease as temperature dependent, occurring mainly between 16 and 29°C. Second, house sentinel fish of known status with the new arrivals so that an infection has something susceptible to reveal itself in. A cold-water quarantine period proves nothing, however long it runs, because the virus can persist latently below permissive temperature and re-emerge when the pond warms in spring. Do not share nets, bowls, siphons, pumps, or wet hands between systems without appropriate disinfection. WOAH identifies all common-carp varieties as susceptible to koi herpesvirus, notes that survivors can remain persistently infected, and requires laboratory methods for defensible detection. KHV signs can include rising mortality, lethargy, separation, gasping, balance change, skin and mucus abnormalities, hemorrhage, fin erosion, and severe gill damage, but none is unique to KHV. Carp edema virus can produce koi sleepy disease, and Argulus can injure skin and transmit other agents; water-quality failures and multiple infections can look similar. Do not heat-cycle fish, salt the pond, or give medications, pesticides, or anesthetics based on an online guess. Test the environment, reduce cross-contamination, contact an aquatic veterinarian, and follow local reporting or laboratory instructions. A dead fish in a multi-fish event should be handled and stored only as the veterinarian or diagnostic laboratory directs, because a useful specimen can be lost through delay or improper freezing.

Housing

A koi pond is life-support infrastructure, not a lined hole with a fountain. Begin with adult biomass and group size. Approximately 1,000 liters per adult is a conservative floor drawn from veterinary guidance, while California Fish Vet notes that a mature female may warrant up to about 1,900 liters. A small social group therefore needs several thousand liters before allowances for displacement, freeboard, filter chambers, future growth, and extreme weather. Calculate actual water volume from measured fill or validated dimensions; decorative labels and pump ratings are often misleading. Use roughly 90–120 centimeters depth only as an initial design context. Colder climates may require deeper stable water and freeze protection; hot climates need shade, gas exchange, and thermal planning. Comply with permits, electrical codes, child barriers, structural requirements, and local wildlife or invasive-species rules. Provide bottom and surface collection, mechanical solids removal, mature biological filtration, circulation without dead zones, and redundant aeration. Select pumps by delivered flow at actual head height, not box maximum. Protect intakes from fish, make overflows screen-safe, prevent a pipe break or siphon from emptying the pond, and route storm overflow away from natural waterways. Keep an emergency air pump, generator or battery strategy, spare critical parts, and a written outage response. Establish the nitrogen cycle before adding koi and increase stocking slowly. Use source-water treatment appropriate to chlorine or chloramine. Test temperature, ammonia, nitrite, pH, nitrate, alkalinity, and dissolved oxygen on a schedule matched to the pond; test immediately after behavior change, storms, outages, treatments, filter cleaning, spawning, algae crashes, or rapid temperature shifts. Winter care protects gas exchange while avoiding dangerous thin ice and unneeded disturbance; summer care anticipates lower oxygen, warmer water, and greater biological demand. Never smash ice over fish or seal a pond completely, and never rely on one heater or one pump without failure planning.

Activity

Koi need continuous opportunities for sustained swimming, turning, depth choice, exploration, and foraging. Provide a broad, unobstructed footprint rather than counting deep but narrow water as equivalent volume. Roughly 90–120 centimeters of depth can be a useful starting context, but depth must support climate stability and safe movement rather than create inaccessible stagnant zones. Arrange returns so the pond circulates waste toward collection points without forcing fish to fight one high-velocity jet all day. Include open lanes, calmer resting areas, shade, and visual refuge from people and predators. Smooth, structurally stable margins and fish-safe planting zones can add complexity, but loose rock piles, sharp wire, exposed pump hardware, narrow gaps, and unstable shelves can trap or abrade large fish. Koi naturally investigate and may uproot plants, so design for that behavior instead of punishing it with a barren concrete box. Enrichment can include dispersed portions of the normal ration, changing but predictable feeding stations, safe floating shade, seasonal natural food opportunities that do not introduce contaminants, and voluntary target or station training. Hand-feeding is optional, not a welfare requirement; it should never involve chasing or withholding adequate nutrition. Judge exercise provision by normal, unimpeded movement and choice, not by how often fish perform at the viewing edge.

Breeding

Koi are externally fertilizing egg scatterers. In suitable seasonal conditions, mature fish may chase intensively and release adhesive eggs onto plants, spawning brushes, pond margins, or other submerged structure. The activity can damage fins and skin, exhaust females, foul water with eggs and milt, and expose adults or eggs to injury in a poorly designed pond. Adults do not provide dependable parental care and may eat eggs and fry. Accidental spawning therefore requires close observation, oxygen and chemistry checks, safe refuge, and removal of hazardous hard edges—not a casual promise to raise every offspring. Planned breeding is a separate aquaculture project: it needs known broodstock health, biosecurity, substantial nursery and culling capacity, protected filtration, multiple grow-out stages, appropriate live or prepared fry foods, and lawful, humane placement for potentially large numbers of fish. Color and pattern are inherited unpredictably, and only a small fraction of fry may meet a breeder's visual goal; that does not make the others disposable. Do not breed fish with chronic deformity, impaired swimming, recurrent disease, or unknown KHV status. Mixing valuable bloodlines without quarantine can spread pathogens through an entire collection. This guide does not give hormone, anesthetic, stripping, spawning-temperature, medication, or culling instructions. Work with an experienced welfare-focused breeder, aquatic veterinarian, and relevant local authority before manipulating reproduction or moving eggs, fish, or water between sites.

Daily Care

Begin with the pond before the food. Confirm that pumps, bottom drains or intakes, returns, skimmers, biological filters, ultraviolet equipment if used, and every air source are operating normally. Look for falling flow, unusual noise, leaks, blocked screens, foam, rapid evaporation, overflowing rainwater, or a water level that could expose a pump. A visual check is not a substitute for alarms or a backup plan, especially during warm nights, heavy feeding, algae die-offs, storms, or grid failures when oxygen can fall quickly. Observe every koi from above and, where possible, from the side. Compare each fish with its own usual posture, swimming effort, social position, respiration, appetite, body condition, skin, fins, eyes, mouth, and gill-cover movement. Watch for isolation, repeated flashing, clamped fins, loss of balance, surface gasping, crowding at an inlet, unusually rapid breathing, lethargy, wounds, ulcers, raised scales, color change, excessive or absent mucus, sunken eyes, or pale and ragged gills. Do not label those signs KHV or parasites from appearance alone. Immediately check temperature, dissolved oxygen where possible, ammonia, nitrite, pH, recent maintenance, chemical exposure, and equipment function; contact an aquatic veterinarian urgently when several fish are affected, deaths occur, respiration is impaired, or abnormalities persist. Feed only after the group is behaving normally. Measure the ration, spread it so timid fish can reach it, and remove persistent leftovers. Record tests, feeding, weather, treatments, additions, mortalities, and equipment work. Trend data reveal a deteriorating filter or seasonal oxygen problem sooner than memory does.

Maintenance

Koi do not need cosmetic grooming. Netting, lifting, scraping, photographing out of water, or handling to improve a show appearance damages the protective mucus layer and increases stress and injury risk. Leave capture to necessary transfer, measurement, diagnostic sampling, or veterinary care. When handling is unavoidable, prepare the destination first, use koi-safe soft nets or a sock net appropriate to the fish, wet all contact surfaces, support the body, protect the eyes and fins, and minimize air exposure. Never suspend a large koi by a standard mesh net or hold it by the mouth, gill covers, or tail. Pond maintenance is the useful equivalent of grooming: remove settled solids before they decay, rinse mechanical stages as designed, preserve biological media, trim unsafe plant material, secure drains, and clean viewing surfaces with fish-safe methods. Do not sterilize the entire mature system during routine care or rinse all biological media in chlorinated water. Separate quarantine equipment visually and physically from resident-pond tools. If algae must be managed, address nutrients, light, circulation, and mechanical removal before adding a chemical; rapid algae death can consume oxygen and destabilize water. Any chemical product must be legal for the setting, compatible with actual pond volume and species, and used only with defensible guidance.

Seasonal Care

Winter and summer are the two periods in which a pond that looks settled can still kill fish, and Penn State extension guidance on pond winterkill and pond water quality describes why. In freezing weather the governing problem is gas exchange: keep an unfrozen opening in the surface using an aerator positioned near the surface or a floating de-icer, so gases from decaying organic matter can leave and oxygen can enter. Never break ice by striking it; the shock concusses fish beneath. Do not run a pump or aerator deep enough to destratify the pond in freezing weather, because that mixes away the warmer deep layer the fish are overwintering in and pulls them into near-freezing water. Through autumn, exclude leaves before freeze-up: net the pond while leaves are still falling, remove fallen material rather than letting it decay under ice, and clear accumulated solids from the filter and pond bottom while the water is still workable. Feeding follows measured water temperature through both shoulder seasons, as described under nutrition, and the Royal Veterinary Center (Macau) koi care sheet is the seasonal veterinary reference used here. In summer the risk reverses: warm water holds less dissolved oxygen while fish metabolism, bacterial demand, and feeding all rise. Add shade, keep aeration running through warm nights, restrain stocking and feeding, watch for algae die-off, and treat a hot still dawn as the highest-risk moment of the day.

Behavior And Enrichment

Koi can learn predictable routines, distinguish feeding locations, approach familiar caretakers, and voluntarily station for observation. Use that capacity to improve care. Pair a distinct visual target or feeding station with a small portion of the normal ration, reward calm approach, and train each fish to pause where its body, fins, eyes, and mouth can be seen. More than one station helps reduce competition and can reveal a fish that suddenly stops participating. Keep sessions short and consistent, count rewards within the normal diet, and end if fish collide, crowd, gasp, or become frantic. Avoid glass or pond tapping, chasing with a net, grabbing, forced jumping, prolonged fasting, or feeding by hand when it causes large fish to strike one another. Training should never mask a welfare problem: unusually eager surface behavior can reflect underfeeding, while reluctance can reflect low temperature, poor oxygen, social exclusion, water chemistry, injury, or disease. Preserve natural foraging and quiet periods rather than turning every human approach into a feeding event. A useful trained behavior is calm transfer into a bowl or sock net under professional direction, but it must be shaped gradually and never used as an excuse for unnecessary capture.

Gallery

Koi — Koi — photo by Laura Porter (Pexels)
Koi — photo by Laura Porter (Pexels)
Koi — Koi — photo by Charles Haacker (Pexels)
Koi — photo by Charles Haacker (Pexels)
Koi — Koi — photo by Mayara Caroline Mombelli (Pexels)
Koi — photo by Mayara Caroline Mombelli (Pexels)
Koi — Koi — photo by Quý Nguyễn (Pexels)
Koi — photo by Quý Nguyễn (Pexels)

Frequently Asked Questions

How large and deep should a koi pond be?
Plan from the adult group, not the juvenile fish on sale. California Fish Vet gives a 250-US-gallon (about 946-liter) practitioner floor per adult koi, and notes that a mature female may warrant up to 500 gallons. These are conservative rules of thumb, not a universal optimum. Include displacement by shelves and rock, filter chambers, freeboard, future growth, and the fact that koi should not be crowded into a nominal volume. A depth around 90–120 centimeters is a common starting context, but winter freezing, summer heating, predators, child safety, soil, groundwater, plumbing, structural loads, and local law may require another design. Favor a broad usable footprint, smooth swimming paths, safe margins, shade, and circulation without stagnant pockets. Have a pond professional review construction and an aquatic veterinarian review stocking before purchase. A large filter cannot turn an undersized body of water into adequate swimming space.
Does clear pond water mean the water is safe for koi?
No. Clarity describes suspended particles and algae, not dissolved oxygen, ammonia, nitrite, pH stability, alkalinity, toxins, or pathogen status. A polished pond can be oxygen-poor at dawn, suffer a pH shift, or carry toxic nitrogen waste that is invisible. Test on a schedule and immediately after behavior changes, storms, outages, spawning, heavy feeding, filter maintenance, algae die-off, or new additions. SRAC gives an operating context of dissolved oxygen at least 5 mg/L, unionized ammonia and nitrite below 0.05 mg/L, and pH near 7. In a home pond, drive ammonia and nitrite as close to nondetectable as the test can reliably show and understand whether the kit measures total or unionized ammonia. Track trends rather than reacting to one isolated color block. Mechanical filtration creates clarity; mature biological filtration, oxygenation, appropriate stocking, source-water treatment, and water changes create chemical stability. Neither replaces the other.
What filtration, aeration, and backup equipment does a koi pond need?
Use staged life support sized for the adult biomass and actual feed load. A bottom drain or other safe solids collection path, skimmer where appropriate, mechanical separation, and accessible waste purge keep organic material from decaying in the pond. A mature biological stage converts ammonia and nitrite; it must remain oxygenated and should not be sterilized during routine cleaning. Returns should circulate the full pond without trapping fish or leaving dead zones. Aeration is a separate function: provide strong gas exchange in the pond and filter, with extra margin during warm nights, heavy feeding, spawning, algae loss, and treatment under professional direction. Size the circulation to a stated turnover convention: pond keeping conventionally targets turning the entire pond volume through the filter roughly once every one to two hours, so a 5,000-liter pond means about 2,500–5,000 liters per hour delivered at actual head height, which usually requires a pump rated well above that figure on the box. Treat that as a husbandry convention rather than a research-established threshold, and as a floor that heavy stocking, warm water, and high feeding rates all push upward. Evaluate pumps at delivered flow after head height and plumbing losses, not the number printed on the box. Design drains and siphons so one failure cannot empty the pond. Keep a tested battery air pump, generator, or other emergency oxygen strategy, plus spare air stones, hose, fuses, and a replacement pump plan. Alarms for power, flow, and water level are valuable, but they do not replace a person who can respond.
What should koi eat, and how should feeding change with the seasons?
Use a fresh, complete koi food as the nutritional base and choose a pellet size every fish can manage. Offer measured portions that the group consumes promptly, distribute them so smaller or less assertive koi eat, and count treats within the total ration. Koi are omnivores, but fruit, vegetables, insects, and pond plants are not automatically balanced. Adjust feeding to measured water temperature, appetite, body condition, oxygen, and filtration—not a fixed calendar date. SRAC's 18–24°C range is a useful active-management context. As water cools, reduce quantity and frequency progressively and follow the food label and local aquatic-veterinary guidance on when to stop; sudden feeding during a brief warm afternoon may be inappropriate if the pond remains cold. In hot water, oxygen capacity falls while metabolism and waste demand rise, so do not feed heavily into poor oxygen or detectable ammonia or nitrite. A fish that stops feeding outside an expected seasonal pattern needs water and health assessment, not a more tempting treat.
Can koi live alone or with goldfish and other pond fish?
Koi are social, active carp and are usually managed in compatible groups, but company is beneficial only when the pond can support the adult biomass. Do not add a second fish merely to solve isolation if doing so overwhelms volume, oxygen, filtration, or quarantine capacity. Goldfish are Carassius auratus and koi are an ornamental variety of Cyprinus carpio: two different species with overlapping but not identical size, feeding, disease, and swimming considerations, and koi reach roughly 90 cm against about 45 cm for goldfish. Large koi can outcompete slow fancy goldfish, while any companion small enough to swallow is unsafe. Every candidate must match temperature, water chemistry, activity, food access, and adult size. More species also expand the pathogen pathways and may complicate veterinary testing or local invasive-species rules. Quarantine each new fish independently before water or equipment is shared. Observe actual access to food and refuge rather than assuming the absence of aggression proves compatibility. A purpose-built koi group with distributed feeding stations is generally simpler and safer than a mixed ornamental collection assembled by appearance.
How should a new koi be quarantined, and can I identify KHV by its appearance?
Use a separate, cycled, covered quarantine system with independent nets, siphons, bowls, pumps, and filtration. WOAH recommends a minimum of four weeks to two months in KHV biosecurity guidance, extended on a risk basis, and attaches two conditions that are the only things making the period diagnostically meaningful. The quarantine must be held at a temperature permissive for KHV expression: WOAH treats water above 16°C as the range conducive to clinical disease and inspection, and describes the disease as temperature dependent, occurring mainly between 16 and 29°C. It should also include sentinel fish of known status housed with the new arrivals. A quarantine run cold proves nothing however long it lasts, because the virus can persist latently below permissive temperature and re-emerge when the pond warms; eight weeks in a 10°C holding tank is not compliance. The exact protocol, temperature exposure, sampling, and laboratory timing should be designed with an aquatic veterinarian, because a period of apparently normal behavior cannot on its own prove freedom from infection. Buy from suppliers with documented health and testing practices, but still quarantine. KHV can cause rising mortality, lethargy, separation, gasping, balance change, mucus or skin abnormalities, hemorrhage, fin erosion, and severe gill damage. Those signs are not unique: low oxygen, ammonia, other infections, parasites, and toxins can look similar, and WOAH notes that survivors may remain infected. Test water and life support immediately, stop sharing wet equipment, and contact an aquatic veterinarian or relevant authority. Diagnosis requires appropriate laboratory methods. Do not heat-cycle, salt, or medicate fish from a visual guess, and do not move exposed survivors to another pond.
How long do koi live, and did the koi Hanako really live for more than 200 years?
Plan for at least a multi-decade commitment. Three cited estimates differ: Royal Veterinary Center (Macau) gives 25–35 years, Smithsonian describes a roughly 40-year average, and California Fish Vet gives a potential of 40–60 years. Smithsonian also repeats the account of Hanako, a koi believed to have lived nearly 230 years based on scale growth rings. That famous estimate is not a controlled population study, is presented as a belief rather than a verified expectation, and cannot tell an owner how long a particular koi will live. Use at least 25–35 years for practical continuity planning, which is the shortest defensible figure rather than the likeliest one; planning short and being wrong means a fish outliving its housing. Pond structure, power costs, seasonal maintenance, veterinary care, fish evacuation during repairs, property moves, disability, and succession should all be considered before purchase. Arrange who will maintain the pond if the owner travels, becomes ill, sells the property, or dies. A dramatic longevity story should raise respect for long-term responsibility, not sell koi as guaranteed two-century pets.
Can unwanted koi be released into a lake or stream, and what seasonal risks should a pond plan cover?
Never release koi, eggs, plants, sediment, or untreated pond water into natural waterways. Carp have been introduced widely, can disturb habitat and water quality, may hybridize with wild populations, and can carry pathogens. Release may also violate local law. Rehome through an experienced keeper, aquatic veterinarian, rescue, dealer, or authority that can legally accept the fish; obtain local instructions for humane disposal after death or disease. Seasonal planning must prevent escape during floods and protect fish through heat, cold, storms, and outages. In summer, shade, circulation, stocking restraint, and redundant aeration help counter lower dissolved oxygen. In winter, maintain safe gas exchange and suitable open water without smashing ice over the fish; avoid a completely sealed surface and review depth and de-icing methods for the local climate. Secure overflow paths and screens so neither fish nor eggs enter drains or streams. The humane solution to an unsuitable pond is managed rehoming, not ecological release.

Recommended for Koi Owners

Koi Food & NutritionBreed-appropriate food, treats, and supplementsKoi Toys & EnrichmentToys and enrichment to keep your Koi engagedKoi Grooming & CareGrooming tools and care essentials for Kois

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Koi Care Summary

Build for the adult group before buying juveniles. Use roughly 1,000 liters per adult only as a conservative planning floor; a small group needs several thousand liters, and a mature female or heavily stocked system may require substantially more capacity. Prioritize footprint and usable swim space, with roughly 90–120 centimeters of depth as a common starting context that must be revised for local winter, summer, predator, construction, and safety conditions. Size mechanical removal for the real solids load and biological media for nitrogen conversion, keep circulation and aeration continuous, prevent a single blockage from draining the pond, and arrange backup power or emergency oxygen for outages. Cycle the system before stocking and add fish slowly. At least daily, confirm pump, filter, air delivery, water level, temperature, appetite, breathing, posture, schooling, and skin or gill appearance. Test ammonia, nitrite, pH, and temperature routinely; track nitrate, alkalinity and dissolved oxygen as the system and local water require. SRAC's operating context of 18–24°C, dissolved oxygen of at least 5 mg/L, unionized ammonia and nitrite below 0.05 mg/L, and pH near 7 is more useful than common-carp survival extremes, but home-pond ammonia and nitrite should be driven toward nondetectable. Feed a fresh complete koi food in measured portions that all fish consume, reducing or stopping according to measured water temperature, appetite, product directions, and professional seasonal guidance—not calendar folklore. Quarantine new koi separately for a minimum of four weeks to two months, extended on a risk basis, under an aquatic veterinarian's biosecurity plan, holding the quarantine at a temperature permissive for koi herpesvirus expression and with sentinel fish of known status alongside the new arrivals. KHV can affect common carp varieties, survivors may remain infected, and signs overlap with other failures; only appropriate laboratory testing can support diagnosis. Sudden deaths, gasping, severe lethargy, loss of balance, ulcers, marked mucus or color change, or pale and damaged gills require immediate water and equipment checks, isolation where safe, and an aquatic veterinarian or relevant authority. Never release koi or pond water into natural waterways.

Sources

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