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Choosing Koi

Koi (Cyprinus carpio) are members of the carp family, the largest family of fish. Unlike their wild cousins, koi are colorful and prized for their exotic appearance and longevity. Koi can grow as large as 3 feet and reach mature length in as little as 3 years. With good care and proper environment, they can live to be 60 years old. Because of their large size, and long life span, koi are best suited to ponds or pools, rather than in-home aquariums.


Koi are cold water fish, thriving best in temperatures between 61 and 75 degrees Fahrenheit. At this range their appetites are keen and they give the best appearance of their colors and patterns. Koi are omnivorous, meaning they will eat a variety of foods, such as insects, worms, snails and plants. Most often they are fed a diet formulated specifically for koi in a pelleted form. Koi become sexually mature at 18 to 36 months. They are egg layers and become interested in breeding as the water temperatures begin to warm in the spring. During this time, they are often very destructive to ornamental plants, so valuable plant specimens may need to be protected or moved. Koi can be aggressive to other fish, even their own smaller offspring. The smaller fish may need to be separated from the larger ones for a time until they are large enough to mix with the older population. Koi can be mixed with larger goldfish or catfish, even a large algae eater.

When you purchase your koi, look for active fish with good appetites. Koi will eat food anytime it’s offered; have the garden center offer food to the fish you are interested in so you can see they are eating. Look for any signs of illness, clamped fins, sores or red spots on the body or fins. Do not buy any fish that appears listless, or is at the surface gasping for air. Sick fish are very hard to treat and can be a constant source of disease for your pond even after they are gone.

Summer Care

During active summer months, feed your fish a good quality diet designed for koi. The most common type is a pellet that is made to float at the surface of the water. Offered at the same time of day and at the same location in the pond, you will be able to observe your fish as they surface to eat. This will give you an opportunity to see your koi, make sure their appetites are good, and look for any injuries or diseases. If you live in an area where wildlife predators are a concern, smaller ponds can be covered with a protective netting to discourage raccoons, foxes, etc. from taking a meal. The netting also serves to keep leaves and other debris from getting into the pond and fouling the water.

Water quality is the single most important factor in the health of your fish. Keep the number of fish in the pond relative to the number of gallons of water your pond holds. When koi are small, you can use the standard 1-inch of fish per gallon of water. This means that 5-six inch fish can live in 30 gallons. But, as koi grow, this ratio doesn’t work well. One 30-inch koi will have a difficult time thriving in a 30-gallon area. In the case of koi, bigger is better. The more space you can provide for your fish, the healthier they will be. If your fish are becoming too large for your pond, either enlarge the pond or try to trade a large koi for a smaller one at local pet stores or garden shops. Watch for overpopulation. If your koi are blessing you with a lot of offspring, it’s time to share with the neighbors. A pond filter will help control the delicate biological balance in pond systems and help control the amount of waste produced by the fish. There are a number of different types available for all sizes of ponds. Keeping a few live plants in the pond will help deter the growth of algae, but choose sturdy, strong rooted specimens as these fish love to disturb plants.

A small fountain or pump to provide movement at the water surface will keep your pond oxygenated. Your local aquatic store can help you test for toxic waste levels in the water such as ammonia or nitrates, or you can use one of the many easy-to-use home kits. When establishing your pond, add only one or two small fish every 2 weeks to avoid overloading the filter with waste products. Remember, your fish are dependant on the pool’s natural filtration system to detoxify the waste they produce. If you add too many fish too quickly, ammonia levels will rise and you may lose your fish.

Winter Care

If your pond or pool is deep enough, your fish can spend the winter outside. Typically, the deepest part of your pond should be no less than 3 feet or 18 inches below the frost line. A heat source should be added to keep the water from freezing. As winter temperatures fall, your fish will begin winter hibernation. Stop offering them food when the water temperature drops to 50 F.

Types of Koi

Koi are classified as metallic or nonmetallic, and differ on the appearance of scales and color patterns. A few examples are listed below:

Bekko – White, red or yellow with black markings
Kohaku – white with red or black
Taisho sanke – white with red or black

Hikarimono – Single colored
Hikari-utsurimono – black with white, red, or yellow