Koi are large ornamental fish that are related to the common carp. Many people think they are a giant breed of goldfish, because they come in similar hues, but they are only distantly related. Koi can grow to impressive sizes, reaching eighteen inches (about half a meter) in a few years. Unlike goldfish, their growth is not constrained by the size of their habitat, so they should never be put in a space too small for them. Koi can live forty to fifty years, and have been known to live more than a hundred years. In Japan, koi can be passed down from one generation to the next.
Koi are classified by their distinctive patterns; a white fish with red markings is called a kohaku, for instance. They are bred for beauty and conformation to certain standards, and a large koi with the right markings can be shown in 'koi shows' and sell for tens, if not hundreds, of thousands of dollars.
Koi ponds are an attractive water feature that can add beauty and serenity to your garden or yard. Many people find that fish watching is a great stress-buster, and koi are very 'personable' fish that can be taught to come take food directly out of your hand.
Be warned, however, that they do need more room than goldfish; if you don't have room for a thousand-gallon (almost four thousand liters) pond that is at least two feet (0.6 meters) deep, don't try to keep koi. Goldfish can grow to a satisfying size in an outdoor pond and can be as soothing to watch, and are significantly cheaper; a one-year old koi sells for anywhere from twenty to fifty dollars and older, larger koi are in the hundreds of dollars.
Koi pond kits may come with a hard shell of set shape and dimensions to be set in the ground, or a waterproof flexible pond liner for a more free-form shape that you design yourself to suit your landscaping. Koi pond kits should also include filtration equipment; the pond must have a system for aerating (adding oxygen) and filtering the water. The koi pond kit should also contain all the tubing necessary, and water treatment chemicals that removes chlorine and adds helpful enzymes and bacteria to make a viable pond ecosystem. For maintaining the health of your fish, you should also have a system for testing water PH levels, and suitable water additives to adjust the PH if necessary.
Commercial 'off-the-shelf' koi pond kits may or may not come with the actual koi. In any event, you may wish to select these yourself, since they are more in the nature of 'pets' than outdoor decor, and you should select the ones you find the most appealing.