IMPORTANT: KOI ARE VERY LARGE POND FISH THAT ARE NOT SUITABLE FOR MOST AQUARIUMS. PLEASE DO NOT KEEP THESE FISH IN AN AQUARIUM UNLESS IT IS 1000 GALLONS OR MORE UNLESS JUST GROWING THEM OUT.
Remarks: Koi (Cyprinus rubrofuscus) are possibly one of the most well-known pet fish in the world. Not only do they capture hearts with their peaceful personalities and assortment of gorgeous colors, they hold a cultural significance as well. The creation of the domestic koi started in 1800s Japan with wild-type Cyprinus rubrofuscus, also known as amur carp, that were being bred as a food fish in muddy pools alongside rice paddies starting to show color mutations. The farmers that owned these fish decided to begin selectively breeding the carp for these mutations to create even more beautiful fish as a hobby. It would not be until 1914 when the first koi were displayed at an exposition in Tokyo that koi began to get noticed by the rest of the world. Ever since then they have become an important symbol in Japan, representing many things such as luck and prosperity as well as becoming a beloved pet kept by those all over the world.
Modern day koi come in a humongous variety of colors and patterns. From an elegant, completely black fish (Karasu) to those with a white body and red markings (Kohaku), there is a full catalogue of different breeds of koi with each color and pattern having a specific name. There are even some with long fins that are commonly called “butterfly koi” in English and "hirenaga koi" in Japanese. While koi are still considered to be Cyprinus rubrofuscus, many are likely hybrids with other types of carp. In fact, certain varieties of koi are known for sure to be hybrids such as the Doitsu line which have large, shiny scales and come from hybridizing Cyprinus rubrofuscus with Cyprinus carpio carpio, also known as the mirror or common carp. Dedicated koi breeders are always looking to produce the best looking and newest koi varieties with many having the goal of winning one of the many prestigious koi shows. At these shows, these fish are closely inspected by judges with only the best of the best winning prizes. Similar to a show dog, offspring of these award winning koi can command a very high price. With coloration, size, health, and lineage all being factors, koi can sell for anywhere from a few dollars to over 1 million dollars USD!
No matter the lineage, grading, or price, koi can be fantastic pets. Not only are they beautiful, but they have fantastic personalities with some becoming close enough to their owners to eat from their hands and be touched. They live a very long time with the oldest known fish on record being a fish known as Hanako who was 226 years old. This is an extreme in regards to the lifespan of koi, but they still can live on average 25-50 years. With this lifespan, they are certainly a commitment and will need this taken into account when considering keeping koi, but it also means you have plenty of time to enjoy and grow closer to your fish.
To ensure your koi thrive and live their full lifespan, it is important that they are cared for properly. While they may start very small, koi can grow up to 3 feet long. This means that to have a proper set up, you will need at minimum 250 gallons of space per koi. Considering they are social and do best when kept in groups, they really should be kept in at least 1000 gallons to be able to have multiple. Their size, high levels of waste production, and social disposition make it where it usually is best to only keep koi in a pond set up. Since koi are very tolerant of a temperature range anywhere from as hot as 90°F to where it is so cold the top of the pond freezes over, they can be kept in an outdoor pond comfortably almost anywhere within the United States.
However, it should be mentioned that koi are most comfortable around 70-75°F and different temperatures mean different care requirements. When it is getting very cold, koi should be fed less and with a specific cold weather diet. In areas where the pond surface will freeze over, they should be gradually weaned to the point of not being fed at all until the temperature rises again. This is because as the temperature drops, so does the metabolism of the koi so they can survive going into a sort of semi-dormant state once the surface freezes. If they go into this dormant state with food in their stomach, it will not be digested and instead will rot inside them resulting in infections and death. On the other side of temperature extremes, when it starts getting around 80°F or warmer, the metabolism of the koi will speed up and they will need to be fed more often. More food and higher temperatures also mean more waste production and less dissolved oxygen. These changes make attention to water parameters and dissolved oxygen levels become more important than any other time of year. To combat this, one can utilize aeration equipment such as fountains as well as pond filtration systems. If pollutants such as nitrates get too high, just like with an aquarium, the pond will need water changes. Bog plants as well as true aquatic species such as lily pads can also aid in water quality however it should be noted delicate plants will be eaten by koi with some being voracious enough to even go after more firm-leaved plants such as Anubias. Adding plants to your pond a few months before your koi can help them get a foothold and fare better against the occasional nibble.
When it comes to tankmates, while koi are peaceful for the most part, they still will eat anything that fits in their mouths. They also lack much of any defense against would be attackers. Both of these aspects along with the fact most koi will be kept in ponds means that tankmates should be selected with caution. An ideal koi companion is a fish that is too large to be swallowed, is docile and not nippy, and enjoys the same water parameters as koi. A few popular options are Chinese high fin banded sharks (Myxocyprinus asiaticus), larger goldfish (Carassius auratus), and adult dojo loaches. Whether you keep koi with other docile pond fish or as a strictly koi only pond, they can still be enjoyed to their fullest for many years with proper care.