The Koi is a carp that was selectively bred originally in Japan for desirable colors. They can get to be very large with 2 feet plus being a common size for adult fish. Because of their large size, they do best in large outside ponds. Each fish needs several hundred gallons of water to adequately care for them. A common mistake is to buy too many fish for your pond. While young, the volume of water in the pond may be fine, but as these fish grow they will need larger volumes of water to prevent growth stunting. Koi health depends on the care and environment condition. One of the most important conditions to keep and care for Koi is to create a special water condition. Besides temperature difference, that is the essential factor to keep this exotic aquarium fish, the other conditions have quite general for other spices character: high efficiency system of water filtration, aeration and bottom cleaning. These ideal water conditions are necessary.
There are many varieties available with some fetching ridiculous prices. The names of various Koi are based on the patterns and colors exhibited by the fish. The Tancho variety is highly sought after having a primarily pure white body with an orange colored pattern on the top of the head. It is a beauty. 

People often confuse goldfish and koi. These fish are similar, both being carps, but koi get much larger and have barbels on the sides of the mouth that they use for foraging and finding food. They also have similar care requirements such as feeding and temperature but Koi need much larger living quarters than goldfish.
The great thing about keeping them is that they can and will develop personalities which makes them great pets. This fish lives for a very long time with a life span of 20 years or more if cared for properly. Proper care means providing large enough volumes of water, feeding high quality foods and keeping the water clean. Clean out the pond filter media often and remove debris from the water surface and the substrate on a regular basis.
Diet
Koi are an omnivorous fish and will often eat a wide variety of foods, including peas, lettuce, and watermelon. Koi food is designed not only to be nutritionally balanced, but also to float so as to encourage them to come to the surface. When they are eating, it is possible to check them for parasites and ulcers. Koi will recognize the person feeding them and gather around at dinnertime. They can even be trained to take the food from one’s hand. In the winter their digestive system slows nearly to a halt, and they eat very little, perhaps no more than nibbles of algae from the bottom. Their appetite won’t come back until the water warms up in the spring