Rasboras or Rasbora heteromorpha are members of a group of small minnow-type fish belonging to the Family Cyprinidae. Currently the genus Rasbora contains around 70 species, which can be found in parts of Southeast Asia (Malaysia, Singapore, Sumatra and southern Thailand), and Africa, although for our purposes we focus specifically on R. heteromorpha. The Greek translation of Heteromorpha literally means differently shaped. Rasboras are commonly referred to as harlequin fish or harlequin rasboras. This reference alludes to the black triangular patch on the back half of their bodies which is reminiscent to the patterns found on the costume of a harlequin.
Rasboras inhabit streams and other watercourses characterized principally by low mineral content and high concentrations of dissolved humic acids. This is a a consequence of those waters flowing through peat swamp forests, where the waterlogged forest soil inhibits the complete decay of leaf litter, resulting in the formation of peat, which leaches humic acids and related compounds into the watercourses. These conditions thus resemble those found on a different continent, namely the black-water habitats of South America.
IN THE AQUARIUM
While most Rasboras live in soft, slightly acid waters in their natural environment, they are tolerant of most aquarium conditions as long as the water is well-filtered and partial water changes are conducted regularly. Rasboras are considered very hardy fish and can tolerate a wider pH range of 5.5 to 7.8, and hardness ranges from zero to 15°dH. The temperature range for Rasboras is around 22°C to 27°C.
Rasboras are small fish. They only grow to an adult size of 3 to 6 cm. Under ideal conditions you can expect them to live up to 10 years of age. They are fast swimming schooling fish and are highly social creatures that function best as a community. They don’t adapt well to a solitary existence. They should therefore be kept in a group comprising a minimum of at least six individuals, though schools of larger numbers are really preferable – not only from the standpoint of the well-being of the fish, but from a point of aesthetics. A large school of Rasboras presents a striking vista even to non-fishkeepers, and their active disposition in the water adds to the spectacle. Being a peaceful species, the Rasboras may be maintained in a community aquarium setup with other similarly sized and peaceful aquarium fishes, including many of the small Tetras, Danios, Pencilfish, Corydoras, Plecos and others.
An aquarium intended to house Rasboras should be well planted with some open areas for swimming.
Most Rasboras are egg-scattering spawners, however, in some rasbora species (such as Harlequin Rasbora), they will deposit adhesive eggs on the underside of the leaves of plants such as Cryptocoryne and Aponogeton, these being among the plants that inhabit the Rasbora’s native waters. The female will swim in an inverted position beneath a chosen leaf, rub her belly along the leaf in preparation for spawning, this action seemingly encouraging the male to join in spawning. When the male joins the female, he adopts a similar inverted position alongside her, and as the female extrudes her eggs and attaches them to the underside of the leaf, the male curls his tail fin around the body of the female and with a trembling motion, emits the sperm that will fertilize the eggs. The fishes repeat this course of action over a period of 2 hours or more, during which a large and well-conditioned female may deposit as many as 100 eggs. However, to end up with fry, it is recommended that the breeding pair be moved to a separate breeding tank.
Distinguishing sexes in rasbora is relatively easy. The male bodies are thinner. Females are more full bodied especially when carrying eggs. The distinct triangular marking on the rear of their bodies differs between sexes. The males have more defined angular markings that extend further back on the lower abdomen than the females.