Name: Cyprinus carpio
Origin: Europe to Japan
Koi are considered one of the last stops for an aquarium hobbyist. It's a hobby that strengthens the aquarium experience to ponds. Koi are being bred worldwide, but the best quality is in Niigata, Japan. Koi can be kept in both tanks or ponds, but they are best kept in ponds. In Japanese, Koi means carp. The term Koi is used to describe carp with one, two or three colours. They are often called nishiki-koi. Koi also get Japanese names based on their colours.
One coloured Koi are; white (shiro-muji), red (beni-goi), yellow (ki-goi), gold (ogon), orange (orenji ogon) and platinum (platinum ogon). Here if the colour designation is followed by ohgon (or ogon) the Koi is metallic.
Two coloured Koi are; red on white (kohaku), black on white (shiro bekko), white on black (shiro-utsuri), black on red (aka-bekko), yellow on black (ki-utsuri) and blue on top with orange or red underneath (asagi). Here bekko means tortoise shell, and utsuri means reflection.
Three coloured Koi are either white with black and red mottles (sanke), or black with white and red mottles (showa). Sometimes sanshoku may replace sanke as it means three colours in Japanese.
The main three varieties (a.k.a. gosanke or the big three) include the kohaku, sanke and showa. These there are more likely to win the Grand Champion of all Koi shows and are loved by most Koi keepers.
The Koi hobby is more like a marriage. Once you have started the hobby, it will stay with you all your life. This is because the average lifespan of koi can reach over 50 years, with the longest lifespans over 100 years and adult size that can reach over 1 meter long!
For one of these I recommend at least a 1000 L tank. Any more of them and you will need a pond. A 2000 L tank or more is best for having a good amount of these fish. Also, filtration needs to be strong. They must be fed low-protein foods in winter, otherwise their stomachs will shut down. In the cold don't feed them a lot. Also, this is not really an aquarium fish. It will probably eat fish smaller than itself. I hope this helps if you consider getting one.
I've kept koi in my 13600 litre backyard pond for approximately 5 years - currently we have 18, with the 3 largest around 60-65 cm each. It gets very cold in the winter here (below freezing 20+ days) and the koi pretty much hibernate over the winter. Once the water temperature gets below 10°C, their stomachs no longer efficiently process food and the ammonia levels will skyrocket and/or you can get a nasty filter crash if you continue feeding them. Ditto in spring - don't start feeding again until the water temperature gets above 10°C; start out slow to get their systems warmed up again. I've successfully overwintered as many as 6 koi in a 265 litre tank with total fish length = 1 meter indoors to see if the babied ones inside were any healthier in the spring than those that hibernated in the pond. I actually think the ones we left in the pond did better. We have also been able to get the fish to spawn and raise babies. I've found that once the fry grow to a size where the adult fish can recognize them as another fish (as opposed to insect larvae or other food) the adults will leave them alone - generally around the 3 cm mark. If a grown fish accidentally sucks up a baby, the adult fish will immediately spit it back out and neither seems any worse for it. Our koi enjoy frozen peas, watermelon, frozen tubifex worms, frozen krill, etc., as treats in addition to regular koi pellets and also keep the pond free of mosquito larvae. If you spend enough time with them, you will learn each fish's personality as they have a bit of a pecking order - there is usually one queen of the pond (almost always a female, and usually the biggest fish in the pond) who is the first to eat, explore, etc. A shy queen will make your whole pond more skittish, so it's worth it to find a reputable breeder in your area and check out the personality of any fish you are interested in before you bring it home.
I own 5 carp fish. They are beautiful fish. They grow very fast in my pond. One is about 40 cm long, the other is 35 cm long, and I also have three Velo Carps that are the same as the original carp, but with long tail, and long dorsal fins. These fish in Japan can be worth 5000 dollars or more. When they get to their adult size, they can lay up to 1000 eggs. These are not very demanding fish, but a good filter is recommended.
Ahh, the Koi Carp. I have had 9 of these in the past, I say nine since 3 of them were snatched up by a heron. These guys get REALLY big, the oldest we have weighs over 66 Kg and has gotten to over 65 cm in length, not bad since we bought it at only 8 cm long! Koi, like goldfish, were bred for ornamental ponds to have bright, attractive colour patterns. They are the same species as all the other carp (Mirror, Leather, Common, Ghost). Ghost carp are one of the most attractive of all the koi carp strains.
I have a big pond in the back yard with hundreds of goldfish and koi. In the top bit of the pond are the adults. When the adults breed in the spring time their little babies fall down the water fall without injury and grow, then when the next spring comes I put the babies in the top or, if they're not big enough, leave them down there and the cycle continues. To ensure I only breed beautiful koi/goldfish I remove the colorless ones and trade them in for more colorful ones so the babies are always beautiful. Once again, nice fish.