The 2,000 or so members of the family Cyprinidae are related to carps and goldfish, and include many staples of the aquarium trade, such as barbs, rasboras, and danios. They range throughout the temperate and tropical waters of Eurasia, North America, and Africa and have been introduced to South America and Australia. All are egg layers, with most scattering the eggs about and leaving them to fend for themselves. The notable exception is the bitterling (Rhodeus sericeus). With the assistance of a long ovipositor, the female deposits her eggs inside a mantle cavity of a freshwater mussel. The male sheds sperm into the water directly above the mussel, relying on the mollusk's breathing process to draw the water into the mantle, whereupon the eggs are fertilized. In return for this favor, the parasitic larvae of the mussel sometimes attach to the female's ovipositor, to be carried away to new homes. Oddly, the bitterling has been introduced into the Bronx River in New York City, where it has apparently found a new host species of mussel in which to breed.

Although cyprinids lack teeth within the jaw, all possess pharyngeal teeth in the throat. These occur at a variety of shapes and sizes and enable members of the cyprinid's family to feed on all manner of foods, ranging from algae to snails.

Native to China, the grass carp (Ctenopharyngodon idella) possesses a voracious appetite and has been introduced into many parts of the world to control evasive aquatic plants. In the United States, trade in this species is limited to sterilized animals that are not capable of reproduction, to avoid the establishment of breeding populations.

Many cyprinids make use of a unique early warning system to avoid danger. When the skin of a cyprinid is broken, during an attack by a predator for example, a powerful chemical is released. School members or fish in the neighboring area flee upon sensing this chemical and thus avoid becoming a meal themselves.

As it relentlessly patrols the aquarium, the body shape and high, angular dorsal fin of this beautiful fish does indeed evoke a sharklike appearance. It is, however, fairly nonaggressive and generally quarrels with members of its own species only if suitable hiding places are lacking. The jet-black body set off by the bright red tail renders it an aquarium favorite.

Growing to a length of 6 inches, the red-tailed black shark prefers water that is slightly alkaline (pH 7.1 to 7.3). It is, by nature, a vegetarian and pre-fers to eat growing algae, but it will also accept algae-based pellets and spinach and other vegetables that have been soaked in hot water. It is native to Thailand and other parts of south Asia. The black shark and other close relatives are also to be found in the pet trade and can be cared for a similar manner.