Buy a koi carp: 12 criteria for assessing its quality and health before paying

Buying a koi often comes down to a few minutes spent leaning over a blue tank, with a seller waiting for your decision. This is precisely when costly mistakes are made: a fish that swims crookedly, a pattern that will not hold, or worse, a disease carrier that will infect the entire pond. Taking ten minutes to go through a precise checklist and ask the seller five questions avoids most unpleasant surprises. Here is the one we use when advising customers, from the edge of the tank to putting the fish into your pond at home.

Prepare Your Pond Before Buying

The first criterion concerns not the fish, but your pond. A perfect koi placed in water that is not ready will become ill within a few weeks. Before you even go to see the fish, check these points:

• The filter is mature: it has been running for several weeks, and ammonia and nitrite tests read zero. A new pond is not ready to receive koi, even if the water is already clear.
• The volume is sufficient to accommodate another fish at adult size, not just at its current size.
• The temperature is suitable: ideally stably above around fifteen degrees.
• A quarantine tank is ready, filtered and aerated, with water similar to that of the main pond.

If your setup is recent, our article on how long to wait before introducing fish into a new pond details the startup phase, and our guide on preparing pond water for koi carp outlines the water quality benchmarks.

Body and conformation: what cannot be corrected

The pattern may change, and the color may intensify. The body structure, however, will not change. That is therefore where you should start. Ask the seller to place the fish in a shallow inspection tank and look at it from above.

The first four criteria on our checklist:

1. Body symmetry: viewed from above, the fish should be straight from head to tail, with no spinal deviation. The sides should be symmetrical. In a female, a broader body is normal.
2. Head proportions: neither too small (often a sign of disrupted growth) nor oversized in relation to the body.
3. Complete fins: pectoral fins of equal size, well spread, with no old tears or bent rays. Dorsal and caudal fins intact.
4. Mouth and barbels: mouth that closes normally, barbels present and symmetrical, opercula closing properly over the gills.

A minor fin defect may be acceptable for an ornamental pond. A deviated spine or deformed mouth is not: these fish often age poorly.

Skin, pattern and color: assessing potential

Aesthetic criteria come next, and these determine the price:

5. Skin quality: shiny, with an even mucus layer. A pure, dense white indicates good development; a yellowish or dull white rarely improves.
6. Color clarity: a thick, even red, a deep black, and clean edges between the colors.
7. Pattern balance: a pattern distributed along the entire length, drawing the eye toward the head.

For a first pond, don't let the pattern hypnotize you. A healthy koi with a simple pattern will bring you more satisfaction than a spectacular fish bought without checking its health. Plain and hardy varieties are also excellent first koi.

Signs of good health to check on site

First observe the fish in its pond, among the others, before catching it. Group behavior says a great deal.

8. Swimming: smooth, within the group. Be wary if the fish remains isolated, motionless on the bottom, stuck at the surface or near the water inlet, or rubs against the sides.
9. The fins: spread out. Fins clamped against the body, especially the dorsal fin, often indicate malaise.
10. The gills and breathing: opercula beating calmly and regularly; if the seller gently lifts an operculum, the gills should be bright red, with no pale spots or deposits.
11. The skin: no wounds, redness, white spots, milky film, filament, or raised scales.
12. The eyes and appetite: clear eyes that are not sunken into their sockets; a fish that comes to eat when a few pellets are distributed.

Also look at the neighboring fish: if a single fish in the sales pond shows worrying signs, the entire batch shares the same water. If in doubt, come back a few days later.

Questions to ask the seller (origin, quarantine, certificate)

A reputable seller answers these questions without difficulty and is not offended that they are asked:

• Where did the fish come from? Country, and farm if possible.
• How long has it been with you? A fish that arrived several weeks ago has already shown its state of health; a fish that arrived the day before has not.
• What quarantine was carried out, for how long, and at what temperature?
• Has the batch undergone a health screening, particularly for KHV?
• What are your water parameters? Temperature, pH, and hardness will help you acclimatize them correctly.
• For valuable fish: is there a breeder's certificate, a reference photo, or a guarantee?

Size, age, and price: finding the right compromise

Breeders refer to young koi by their age: tosai for a fish in its first year, nisai for the second, and sansai for the third. Price follows size, but also certainty: the older the fish, the more established its pattern, and the lower the risk you take.

It is better to buy fewer fish, but of better quality and in good health. An overcrowded pond degrades water quality and undoes all the effort put into selection. To determine your population size, our article on the pond volume required for koi provides general guidelines.

Transport, bagging, and acclimatization

The fish is selected, paid for, and bagged. The hours that follow matter just as much as the choice itself.

1. The bag: approximately one-third water and two-thirds oxygen, double thickness for large fish. For very large koi, suitable transport bags or boxes are preferable.
2. The journey: lay the bag flat or secure it in a box or cooler, protected from light, heat, and shocks. No detours: go straight home.
3. Floating: place the sealed bag on the water in the quarantine tank, in the shade, for twenty to thirty minutes to equalize the temperature.
4. Water acclimation: if the pH or hardness values differ significantly, gradually add a little tank water to the bag over half an hour.
5. Transfer: remove the fish with a net or sock and place it in the tank. Do not pour in the water from the bag, which contains transport waste and possibly pathogens.

A suitable water conditioner can help reduce transfer stress. Keep the tank covered with a net: a stressed koi will readily jump. Do not feed it on the first day, then resume feeding small amounts.

Quarantine on arrival: non-negotiable

This is the step people are most tempted to skip, and it is the most important one. A fish that appears perfect may carry parasites, bacteria, or a latent virus that only manifests after the stress of transport. Quarantining it for three to four weeks in a separate, filtered, aerated tank at a stable and relatively warm temperature helps reveal these problems before they affect your entire pond.

During this period, observe the fish every day, test the water regularly, and note what you see. A salt bath can be useful in some cases; our article on quarantine salt baths explains how to carry one out safely. A common case: a hobbyist introduces two new koi directly in spring “because they looked in great shape,” and three weeks later sees the entire pond rubbing itself against surfaces. Quarantine would have limited the problem to a tank of a few hundred liters.

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Once the quality has been assessed, the question remains whether the price is fair: How to estimate the price of a koi carp?.

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Frequently asked questions

— How can you tell whether a koi is healthy before buying it?

Observe it in the group: smooth swimming, fins held open, calm breathing, red gills, shiny skin without wounds or film, eyes that are not sunken, and interest in food. Also look at the seller's pond's overall condition.

— What size koi should you buy to get started?

Fish measuring 15 to 30 cm are a good compromise: reasonable price, pattern already discernible, and good ability to adapt. Very small fish are still a gamble, while large fish require a substantial budget and pond.

— Can a new koi be placed directly in the pond?

It is not recommended. Even a fish that appears perfectly healthy may carry parasites or a virus. Quarantine for three to four weeks in a separate, filtered tank protects the fish already in place.

— What is the best season to buy koi?

Late spring and early autumn, when the water remains above about fifteen degrees for a sustained period without becoming scorching hot. The fish are active, adapt better, and quarantine is more reliable.

In summary

• First assess the seller's pond: a single sick fish in the tank is a warning sign for the entire batch.
• Conformation and health before pattern: a twisted body or abnormal swimming cannot be corrected.
• Ask the key questions: origin, arrival date, quarantine, screening, water conditions.
• Transport them in an oxygen-filled bag protected from light, acclimate them slowly, and never pour the bag water into the pond.
• Quarantine for three to four weeks in warm, stable water: it's the best protection for your other fish.

Last updated: October 3, 2026.