Koi, or domesticated ornamental carp, are among the most widely recognized and desirable pet fish in the world. The beautiful markings, bright colors, sleek shapes, and graceful motions of Koi have attracted multitudes of enthusiastic admirers for generations. These "living jewels," as the Japanese call them, add an element of elegance and interest to any garden setting. Domesticated Koi were bred from common carp, which were originally kept as food fish in ancient China. Normally a uniform silver, grey, brown, or greenish-yellow in color, wild carp occasionally exhibit colored patches, streaks, or spots, which is caused by a genetic mutation.

Beginning more than a thousand years ago, people in Asia began breeding captive carp chosen for their unique natural markings. In the 1820s, these fish found their way to Japan via trade. It was the Japanese who first called them "Koi," which simply means carp. They refer to the ornamental varieties as "Nishikigoi" or "brocaded carp." This name refers to the lovely patterns and vibrant colors of Koi, which are reminiscent of the lavish designs and sumptuous texture of woven brocade fabric. The popularity of Koi soared in Japan after a public exhibition in 1914, and breeders expanded their efforts to create new varieties.

This selective breeding process eventually resulted in the many types of ornamental carp we know today. Varieties are identified by differences in color, pattern, and scale type (or lack of scales). Koi come in a range of colors, including white, red, orange, black, blue, and yellow. They may be a single color, or a combination of these colors.

The most popular type of Koi are called Gosanke, a classification which includes three breeds: Kohaku, Sanke, and Showa. The oldest type is Kohaku, which have white skin with red markings. Sanke and Showa have red and black markings on a white ground. Some Koi have a metallic sheen, or sparkling scales. Prize Koi possess striking coloration in crisp, well-defined patterns that have a flowing, artistic quality. Excellent specimens of rare varieties are treasured by collectors, and can command high prices.

Many people believe the finest Koi still come from Japan, especially the Niigata Prefecture, where a number of the most famous varieties were first developed. The wild carp from which Koi are descended are hardy, adaptable, and long-lived fish, and domesticated Koi retain these characteristics. Even though they can survive in less than ideal conditions, care must be taken to maintain a satisfactory environment and diet to preserve Koi's bright colors, good health, and longevity.


Koi are cold-blooded, which means their body temperature is determined by the water temperature. This is why the activity level of these fish varies seasonally. Koi grow most rapidly, are most active, and eat the most in the summer. In the fall, Koi begin to swim less rapidly. Their metabolism rate also slows down, and they require less food. It is essential to stop feeding Koi as soon as outside temperatures fall below 50 degrees Fahrenheit, even if the fish appear hungry. Once their metabolism shuts down for winter Koi are unable to digest food; it will go bad in their stomachs, causing discomfort and possibly illness. During the winter, Koi become very sluggish, and eat next to nothing. They spend the winter months swimming very slowly at the bottom of the pond, where the water is warmest. Then, as the weather warms up in the spring, these "swimming flowers" will rise to the surface of the pond again, to join the other colorful blooms adorning the garden.

Koi Growth

A Koi's growth is not hindered by the size of the pond that it is in. They may reach up to three feet in length, and are capable of outgrowing a very small pond. It general, it is recommended that a Koi pond contain a minimum water volume of 500 gallons, with at least 150-250 gallons per fish. The more room they have to freely swim, the happier and healthier these fish will be.

When provided with the proper environment, Koi make delightful pets. Children and adults alike find these fish fascinating to watch, and relaxing to spend time with. Their distinctive markings make them easy to tell apart, and each has a distinct personality that makes it simple to come up with nicknames. Koi are sociable creatures who will willingly share a pond with similar fish. They tend to swim in small groups, though unlike many fish they do not form schools. Koi like people, too. They learn to recognize the people who feed them, and will gather to meet them at feeding time.

Predators of Koi

Koi are not only attractive to people, but to predators as well. Wildlife such as raccoons, foxes, and weasels, fishing birds such as herons and kingfishers, domestic cats, and even dogs present a threat to Koi. For this reason, it is important to design a Koi pond with adequate safe areas for the fish to hide. Water plants in the pond provide shelter, and overhanging trees help screen the Koi from birds above, as well as provide cooling shade. Sunken structures that allow the fish to swim inside also offer good protection. Ponds of sufficient depth with steep sides prevent wading birds from standing in the water to snatch precious Koi, and also make it more difficult for raccoons to grab them.

Koi Pond Construction

The ideal depth of a Koi pond depends on the climate. In areas where temperatures never fall below freezing, Koi can be comfortable year round in a pond three or four feet deep. In areas where the weather is cold enough to freeze the surface of the water, a pond depth of no less than four or five feet is necessary to provide a warm enough place for Koi to wait out the winter. The best pond construction materials and methods depend on budget, site, and installer skills. The prepared hole may covered with a simple liner, a rigid plastic pool form, or permanent concrete basin. A water filter or pond skimmer may be necessary to help keep the pond water clear and maintain the cleanest, healthiest home for the fish. For specific recommendations, it is best to consult a professional water garden designer or Koi expert.

Water Chemistry

Once the pond is up and running, the chemical composition of the water must be checked on a regular basis. Even water that looks clear can contain harmful contaminants, such as chlorine, ammonia, nitrite, or pesticides. Monitoring water quality is especially important in the summer when Koi produce the most waste, which can create gases toxic to fish. In addition, water temperature and pH level (acidity or alkalinity) should be tested, using one of the numerous water quality test kits available.

While it is vital to understand the basic needs of Koi before beginning, it is not necessary to have a large property or budget to get started with this rewarding hobby. Taking the time to plan ahead for their comfort, and consistently following a few simple rules, helps ensure that Koi can continue to beautify their surroundings, and be charming companions, for many years to come.