Koi are large, ornamental varieties of the Amur carp. They are massive, long-lived pond fish that produce an extreme amount of waste. Highly social and intelligent, they can learn to recognize their owners. Due to their immense adult size, they are entirely unsuitable for indoor aquariums and require a very large pond with heavy filtration for their adult life.
At a Glance
Tank Requirements
Water Parameters
Temperament & Compatibility
Available Varieties
Each variety inherits the species defaults above. Only differences are highlighted.
Standard Adult
The standard adult Koi. Requires a deeply excavated pond to survive winter freezes and massive biological filtration to handle their bioload.
Same care requirements as the base species
Also known as: Gosanke, Kohaku, Adult Koi
Juvenile (Grow-out)
Young Koi commonly sold in pet stores. They grow exceptionally fast. While they can temporarily be housed in large aquariums, they must be relocated to a pond within their first year.
Differs in 3 traits from base species
Also known as: Baby Koi, Small Koi
Butterfly Koi (Long Finned)
A radical morph selectively bred for elongated, flowing fins and barbels. They are slightly slower swimmers than standard Koi and their fins are easily snagged on sharp rocks or pond hardware.
Differs in 2 traits from base species
Also known as: Dragon Carp, Longfin Koi
Koi Care Summary
The Koi, scientifically known as Cyprinus rubrofuscus, belongs to the Cyprinids family group. Adults reach a maximum size of approximately 80 cm and have a fusiform body shape with a fast growth rate. Classified as peaceful, this species primarily occupies the surface, mid-level, and bottom zones of the aquarium and produces a extreme level of biological waste.
To thrive, the Koi requires a freshwater only aquarium with a minimum volume of 3785 litres and a tank length of at least 300 cm. Water temperature should be maintained between 4°C and 28°C, with a pH range of 6.5–8.5 and a general hardness of 4–15 dGH. A moderate water flow is preferred. To replicate their natural habitat, the tank should include sand (sifters) and smooth gravel (sensitive barbels).
This species should be kept in groups of at least 3 to reduce stress and encourage natural behaviour. Keepers should be aware that this species may uproot or eat live plants and is a known jumper, so a tight-fitting lid is essential. This species is available in 3 varieties: Standard Adult, Juvenile (Grow-out), and Butterfly Koi (Long Finned). Each may differ slightly in size, temperament, or care needs, so it is worth checking the specific requirements of the variety you plan to keep.
Similar Species
Giant Gourami
Osphronemus goramy
The Giant Gourami is a massive, highly intelligent anabantoid that behaves more like a wet pet than a typical community fish. They grow extremely large and have voracious appetites, often completely consuming any live plants in their tank. Due to their immense adult size and extreme bioload, they require massive aquariums and robust filtration.
Amano Shrimp
Caridina multidentata
A highly popular, industrious freshwater shrimp renowned for its unmatched algae-eating abilities. Popularized by aquascaper Takashi Amano, they are significantly larger and hardier than most dwarf shrimp (like Cherry Shrimp). They are notorious food-thieves during feeding time and excellent escape artists, requiring a tight-fitting lid. Their larvae require saltwater to develop, so they will not overpopulate in a freshwater aquarium.
Goldfish (Comet / Common)
Carassius auratus
One of the most commonly kept and misunderstood fish in the hobby. Single-tailed goldfish (Comets, Commons, Shubunkins) are fast-growing, highly active coldwater fish that produce enormous amounts of waste. They are best suited for ponds or massive aquariums. They will uproot plants, sift substrate constantly, and eat anything that fits in their mouths.
Guppy
Poecilia reticulata
One of the most iconic and widely kept aquarium fish. Renowned for their incredible colors, hardiness, and prolific breeding. As livebearers, they give birth to fully formed, free-swimming fry. Males constantly pursue females for mating; therefore, a ratio of at least 2 females per male is strongly recommended to disperse stress.
Flowerhorn Cichlid
Cichlasoma sp. hybrid
A man-made hybrid cichlid famous for its vibrant colors, extreme interactive personality, and the large nuchal hump (kok) on its head. They are incredibly aggressive 'wet pets' that will attack hands, tank mates, and equipment. They have a massive bioload, dig constantly, and must almost always be kept completely alone.
Red-Tail Catfish
Phractocephalus hemioliopterus
A massive, fast-growing predatory catfish from the Amazon basin. Famous for its voracious appetite, bulldozer-like behavior, and distinct red caudal fin. Due to its enormous adult size (often exceeding 4 feet), it is strictly a 'monster fish' that requires a heated indoor pond or a colossal custom aquarium. It is highly predatory and will swallow absolutely any tank mate that can fit into its cavernous mouth.