Dr. Kristin Claricoates, DVM

Natural history
Koi originated as domesticated versions of the common carp.  The common carp’s natural habitat is wide in range, including Asia and Central Europe.  The common carp was domesticated and initially used as a food source due to the fish’s ability to survive in a wide range of temperatures.  The Roman Empire further spread the knowledge of this fish as a food source while it was still in power.  They were eventually bred for their beauty and color in the early 1800s in Japan, which spring boarded them into worldwide recognition at the dawn of the 1900s after an exposition in Tokyo.  In Japanese, the word “koi” simply means “carp”.  The lineage and genetics of koi are uncertain, but it is thought that there are two subspecies of carp.  One subspecies is derived from Western Eurasia (Cyprinus carpio carpio) and another is from East Asia (Cyprinus carpio haematopterus).  Today, there are many varieties of koi, which are distinguished from one another by coloration, patterning, and scalation.

Unlike goldfish, which are now a separate species from their parent species the Prussian Carp, Koi are considered subspecies of the common carp and after just a few generations, can return to a normal wild type common carp.  Additionally, while Koi and goldfish could mate with one another, their offspring would be sterile.  In addition, Koi have prominent barbells on their lip, unlike the goldfish.

In many man-made bodies of water in North America, koi are used to keep water born insects under control.  However, they are thought to be an invasive species in most natural bodies of water as they can be detrimental in several ways.  Because they stir up the water, they reduce the number of aquatic plants, thereby reducing the oxygen in the water.  They can make water unsuitable for drinking—even for livestock!  For this reason, we do not ever recommend releasing koi into natural bodies of water.

Koi should be fed twice daily or less, depending on the temperature of the water.  Only give them an amount of food that they are able to eat within 5 minutes each feeding time.

Here is a good rough guideline as to how often they should be fed: Between 72 and 82 degrees Fahrenheit, feed up to two times daily.  Between 65 and 70° F, feed once daily.  Between 53 and 65° F, feed only every other day.  Do not feed below 53° F.
If they are overfed in summer, there can be an increase in the bacterial population resulting in poor water quality.

It is recommended that Koi live in large ponds or tanks.  Koi ponds should be stocked according to this rule- there should be 1 fish for one square foot of pool surface area.  In established ponds, the rule of thumb is for every 2-3 inches of the length of koi to have a square foot of pond surface area (Example- two 6 inch long koi require at least 4 square feet of pond surface area).
Koi require at least 3 to 4 feet of depth in a pond with at least 1,000 gallons of water.  The pond should have excellent water quality, natural plants, rocks, a rocky hangover area where fish can hide from natural predators, and a smooth gravel substrate.

It is important to have waterfalls to help keep the water moving as well, as a source of introducing more oxygen into the water, and to have a good filtration system.  A good filter system means there is less maintenance to do on the pond, but a cheap filter system will be more expensive in the long run as it will require a lot of upkeep and can potentially harm or kill your fish.  Koi clay is also something you can add to your pond which has lots of minerals that can be added to the system and koi love it. Also, koi clay is good at killing string algae as an added benefit.

To introduce your new koi fish into a pond or a tank, equalize the temperature of the bag of water to the pool or aquarium, then transfer the fish by hand.  The water your fish was transported in should never be added to the tank or pond your fish is transferred to as it may have free swimming parasites (some stages of parasites are not on the fish but swimming in the water).  After taking your fish out of the transportation bag and prior to transferring to a quarantine pond or tank, place your fish in a salt dip for 10-15 seconds.  This salt dip should be 1 pound of non-iodized salt per 1 gallon of water- again, only to be used for 15 seconds as it is a very high salt concentration.  Then, you can introduce your fish into the quarantine pond or tank.

The quarantine tank can be a lot smaller than a pond because it will only be a temporary home for the Koi.  Most quarantine tanks are 100 to 500 gallons in size.  The quarantine setup should include a filter, air pump, heater, net tank, and cover. If you have other fish already in your pond, do not introduce this fish into the pond just yet.

Ideally, your new fish should be quarantined for at least four weeks in a tank with a temperature between 74 and 76° F.  Many parasites have a life cycle of 10-14 days, and it is for this reason we recommend a quarantine of at least 3 weeks.  In this quarantine, you should have a 1% concentration (Example: 1 cup per 100 gallons) of non-iodized salt per water.  A broad spectrum parasiticide can be used in the tank as well.  During this quarantine, it is ideal to test the water daily for temperature, pH, Ammonia, Nitrate, and salinity.  Also, do not feed your new koi for a few days until the koi is accustomed to its new surroundings.

During the quarantine, pay attention to the behavior of your new koi.  If it is scratching its side against the tank or materials within the tank, it may have a parasitic infection.  Should you see scratching, we recommend you come to the hospital to test for parasites, which will be treated according to the parasite found.  If you have purchased only one koi, it is not uncommon to find them near the bottom of the tub or pond.  However, if it is near the bottom of the tub or pond with its fins clamped, this may be another sign of a parasitic infection.  During the quarantine, do a 25% water change each week, which can be done all at once or by changing a small amount of water every day.  Remember to have that water salted with 2 tablespoons of salt per 5 gallons of water to prevent diluting the salt in the tank.

After the three-week isolation period is over, prior to placing the koi in your communal pond or tank, consider testing for KHV (Koi herpes virus).  It is a blood test or PCR swab that will let you know if the Koi fish has ever been exposed the the virus or not.  If your koi is positive for KHV, placing it in your pond puts all of your fish at risk, and you can lose almost all of your fish population in only days to weeks.  If you did not perform a KHV test, but you feel that the Koi is healthy, you may move the Koi out to the pond if the quarantine period is over.  Remember, if your koi looks sick, do not introduce it into your pond even if the 4-week period is over.
If quarantine is not possible, you risk infecting your other fish with any parasites your new fish may have.  To minimize the risk of infectious diseases being transmitted, we recommend the entire pool be treated with a broad spectrum parasiticide every 3 days for 3 treatments.  This is not as reliable a method to prevent contamination of your other fish from any diseases your new fish may have.  This may be harmful to your plants.

Diseases of Koi/ common medical problems​

  1. Algae “Green water”: One of the most effective ways of combating single celled algae is an in-line UV sterilizer as part of your filtration system.  It is an excellent and non-invasive way of dealing with certain types of algae (and harmful bacteria for that matter) that can easily be added to your existing piping.
  2. Parasites: Koi are particularly vulnerable to parasites in the spring when their immune system is not yet at full strength, particular to gill flukes.
  3. Koi Herpes Virus: No treatment exists for either this disease and introducing a fish with KHV into fish stock can quickly wipe out the fish within a pond.