Koi is an informal group of the colored variants of the Amur carp. Several varieties are recognized by the Japanese. Koi varieties are distinguished by coloration, patterning, and scalation. Some of the major colors are white, black, red, orange, yellow, blue, and cream.Carp are a large group of fish originally found in Central Europe and Asia. Various carp species were originally domesticated in East Asia, where they were used as food fish. Carp are coldwater fish, and their ability to survive and adapt to many climates and water conditions allowed the domesticated species to be propagated to many new locations

The goldfish (Carassius auratus) is a freshwater fish . It is one of the most commonly kept aquarium fish.Goldfish have strong associative learning abilities, as well as social learning skills. In addition, their visual acuity allows them to distinguish between individual humans. Owners may notice that fish react favorably to them (swimming to the front of the glass, swimming rapidly around the tank, and going to the surface mouthing for food) while hiding when other people approach the tank. Over time, goldfish learn to associate their owners and other humans with food, often "begging" for food whenever their owners approach.

Oscars will often lay claim to an area of the aquarium and will be very aggressive towards other fish encroaching on their newly established territory inside the aquarium or lake. The size of the territory varies depending on the size and aggressiveness of the fish based on its surroundings. Once the oscar establishes a territory, it will vigorously defend it by chasing away other fish.Although the species is widely regarded as sexually monomorphic, males have been suggested to grow more quickly, and in some naturally occurring strains, males are noted to possess dark blotches on the base of their dorsal fins. The species reaches sexual maturity around one year of age, and continues to reproduce for 9–10 years. Frequency and timing of spawning may be related to the occurrence of rain.ocellatus fish are biparental substrate spawners, though detailed information regarding their reproduction in the wild is scarce.

The guppy (Poecilia reticulata), also known as millionfish and rainbow fish, is one of the world's most widely distributed tropical fish, and one of the most popular freshwater aquarium fish species. It is a member of the family Poeciliidae and, like almost all American members of the family, is live-bearing.Guppies, whose natural range is in northeast South America, were introduced to many habitats and are now found all over the world. They are highly adaptable and thrive in many different environmental and ecological conditions. Male guppies, which are smaller than females, have ornamental caudal and dorsal fins, while females are duller in colour.

The neon tetra (Paracheirodon innesi) is a freshwater fish of the characin family (family Characidae) of order Characiformes. The type species of its genus, it is native to blackwater and clearwater streams in the Orinoco and Amazon basin of South America.Its bright colouring makes the fish visible to conspecifics in the dark blackwater streams,and is also the main reason for its popularity among tropical fish hobbyists.The neon tetra has a light-blue back over a silver-white abdomen. During the night, the blue and red become gray or black as the fish rests—it reactivates once it becomes active in the morning. It grows to approximately 3 cm (1.2 in) in overall length. Sexual dimorphism is slight, with the female having a slightly larger belly, and a bent iridescent stripe rather than the male's straight stripe. They have recently become available in a long-fin variety.

The Siamese fighting fish (Betta splendens), commonly known as the betta, is a popular fish in the aquarium trade. Bettas are a member of the gourami family and are known to be highly territorial. Males in particular are prone to high levels of aggression and will attack each other if housed in the same tank. If there is no means of escape, this will usually result in the death of one or both of the fish. Female bettas can also become territorial towards each other if they are housed in too small an aquarium. It is typically not recommended to keep male and female bettas together, except temporarily for breeding purposes which should always be undertaken with caution. Colours available to the aquarist include red, orange, yellow, blue, steel blue, turquoise/green, black, pastel, white ("opaque" white, not to be confused with albino) and multi-coloured fish.

Platy is a common name of freshwater fish in the genus Xiphophorus that lacks a "sword" at the bottom of their tails. Both species are livebearers, similar to other fish of the family Poeciliidae, such as the guppy and molly. Platies are native to the east coast of Central America and southern Mexico.The platy (Xiphophorus maculatus) grows to a maximum overall length of 7.0 cm (2.4 in). Sexual dimorphism is slight, the male’s caudal fin being more pointed. The anal fin of the male fish has evolved into a gonopodium, a stick-shaped organ used for reproduction. The female southern platyfish’s anal fin is fan-shaped.The variatus platy (Xiphophorus variatus) grows to a maximum overall length of 7.0 cm (2.8 in). In the wild they are olive in color with black marbling or spots on the side of the caudal peduncle.