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Koi

Fish

The living jewels of the pond world that can genuinely outlive their keepers!

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Did you know? Koi recognize individual people and will learn to approach a familiar keeper at the pond's edge for food. Long-term keepers describe fish that surface for one household member and ignore another, which is a considerable amount of animal for something sold as pond decoration.

The first week

The numbers a new owner needs in the first week, each from the article it links to. Rows with no link come from the encyclopedia.

Pond volume: Roughly 250 gallons per adult, or 10 gallons per inch of fish. 1,000 gallons is the cited practical minimum and supports only a few fish; size for the adults they become, not the juveniles you bought.Setup guide →
Depth: 3 feet minimum, 3 to 5 feet or more preferred. Depth buys temperature stability, predator protection and survival through a frozen winter.Setup guide →
Indoors is temporary: An aquarium can hold a juvenile. Matching an adult's volume and filtration indoors is impractical for most keepers, which is why the outdoor pond is the standard.Setup guide →
Temperature: Comfortable at 64 to 75°F, growing best across 59 to 77°F, and able to live through far wider extremes. The real risk is a rapid swing of more than about 2°F a day, not the absolute number.Setup guide →
Filtration: Oversized filtration with a UV clarifier and strong aeration, which is where most beginner ponds fall short. No UVB: the UV unit treats the water, not the fish.Setup guide →
Predators: Herons and raccoons are real threats. Netting sized to the pond, and depth.Setup guide →
How many: A single koi is a poorer arrangement than a group, and the constraint is pond volume, not compatibility.Enrichment guide →
Feeding by temperature: Stop below about 48 to 50°F. Sparingly once or twice a day from 50 to 68°F, 2 to 4 times a day from 68 to 86°F, and cut back again above 86 to 90°F.Feeding guide →
Why cold feeding is dangerous: Koi have no true stomach. Food left in the gut as the fish enters cold torpor lets bacteria cross the intestinal wall into the blood.Feeding guide →
Portion and food: What is cleaned up in 3 to 5 minutes, the easiest rule to overfeed on; 1 to 4% of body weight a day is the precise alternative. Wheat-germ formula in the 50 to 64°F range, growth formula above about 60 to 64°F.Feeding guide →
Handling: As little as possible. Guide the fish into a floating bowl with a net rather than lifting it, with wet hands: dry surfaces strip the slime coat.Handling guide →
Quarantine: A strict 4 to 6 weeks or longer for every new fish. It is the prevention for koi herpesvirus, which kills 80 to 100% in an outbreak, has no cure, and leaves survivors as lifelong carriers.Health guide →
Spring restart: A dose of beneficial bacteria at startup, with the oversized filtration and UV, prevents more of the health list than any treatment.Health guide →
Budget, the fish: $10 to $100 for a pond-quality juvenile, often $10 to $50 for a 4 to 6 inch fish. Imported Japanese koi average $100 to $1,500, show quality $1,200 to $15,000.Cost guide →
Budget, the pond: An installed residential pond commonly runs $5,100 to $15,875, small prefab setups from around $500, and $1,000 to $2,000 or more a year to run. Aquatic vets are uncommon; a routine exam, where one exists, runs in the low to mid hundreds.Cost guide →
Lifespan: 25 to 50 years with good care; some domestic lines average closer to 15.Cost guide →
Adult size: Typically 24 to 25 inches (about 60 cm), up to 3 feet (90 cm) in a spacious pond.
Power outage: Oxygen loss is the threat, not darkness, and the same three problems, oxygen, temperature and a dead filter, govern moving a fish too.Power outage guide →

Emergency card

Test the water first, then act on these today. Involve a vet if one is available.

Gasping combined with high ammonia
Rapid, widespread mortality, or flashing and sores from the common parasites: ich, Costia, Trichodina, flukes, anchor worm, fish lice
Severe open ulcers
Suspected KHV: gill necrosis, excess mucus, lethargy, skin lesions. Emergency and culling-level serious

Correct water quality immediately. For valuable fish or any unclear diagnosis lean toward professional guidance, though aquatic vets are limited and many keepers rely on koi-specific resources.

From Common Koi Health Issues (Water Quality Is the Root of Almost Everything) →

Where to go next

How Much Does Koi Fish Ownership Really Cost?

The fish from $10 to $50,000, what a pond actually costs to build, the yearly run rate, and a lifespan that outlasts most mortgages.

Koi Pond Setup: Built for Waste Volume, Not Just Water Volume

Volume, depth, temperature, substrate, and the filtration sizing that decides whether the pond works.

Koi Feeding: Schedule, Diet, and Why They Stop Eating

The temperature-banded feeding schedule, seasonal protein, portion rules, and why koi stop eating.

How to "Handle" Koi (Mostly, You Don't)

Why handling stays minimal, the net-into-a-floating-bowl method, and the one variety that acts like a dog.

Common Koi Health Issues (Water Quality Is the Root of Almost Everything)

Parasites, bacterial ulcers, water quality crises, and koi herpesvirus, split into what is an emergency and what is not.

Koi Enrichment: Volume, Depth, and Somewhere to Overwinter

Volume then depth, stocking for the adults rather than the juveniles, and what the fish literature does and does not support.

What to buy

Outdoor pond of at least 1,000 gallons, larger strongly preferred

EPDM rubber liner and underlayment

A deep zone of 3 feet minimum, 3 to 5 feet or more preferred

Oversized mechanical and biological filtration

UV clarifier or sterilizer

Pump sized to turn the full volume over once or twice an hour

Aeration kit or waterfall pump

Predator netting

De-icer, in climates that freeze

Seasonal koi pellets, wheat-germ and warm-season growth formulas

Water test kit

Beneficial bacteria, for spring startup

Water conditioner

Pond net and a floating bowl or soft container

A separate quarantine system for new arrivals

Underlined items are paid Amazon links. We may earn a commission at no extra cost to you.

Prices and the reasoning behind each item are in How Much Does Koi Fish Ownership Really Cost?.

Full Koi gear list

Frequently Asked Questions

How big should a koi pond be?

Plan on roughly 250 gallons per adult, or 10 gallons per inch of body length. The commonly cited practical minimum is 1,000 gallons, which supports only a few fish, so go larger for water stability and healthy growth.

Why do I need to stop feeding koi in cold water?

Koi have no true stomach, food passes straight into the intestine. If a koi enters cold-water torpor with food still in its gut, undigested material can let harmful bacteria cross the intestinal wall into the bloodstream. Stopping feeding before it gets that cold isn't just a rule of thumb, it's the actual reason koi ponds get into trouble in winter.

What is koi herpesvirus (KHV) and how serious is it?

The most serious koi disease, causing gill necrosis, excess mucus, lethargy, and skin lesions. Outbreaks can carry mortality rates of 80 to 100%, it's temperature dependent, and there's no reliable cure. Survivors can remain carriers, which is exactly why strict quarantine of 4 to 6 weeks or longer is the critical prevention step.

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Koi

Cyprinus rubrofuscus

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