These great little colorful fish have been in the hobby for more than 100 years, believed to have been introduced to the aquarium hobby in 1906. Male and female harlequin rasboras have the same coloration – a magnificently bright orange color (other fishkeepers have described this color as pink) on the top and silver below. They have a trademark triangular black marking (sometimes described as blue-black) on each side that begins below the last half of the dorsal fin and tapers off into a line that ends at the end of the caudal peduncle. Harlequin rasboras have tinges of orange on their dorsal, caudal and anal fins, as well. They are small, stocky fish with large eyes. Females are more plump than the males.
Harlequin rasboras are good community fish that will not harass others. They loosely shoal together and do best in groups larger than six. A large shoal of harlequin rasboras is especially impressive in a densely planted aquarium with enough open space for swimming. They will establish a pecking order among themselves, which often results in nipped fins of less dominant individuals. Nipped fins heal quickly. Keep a small group in at least a 10-gallon aquarium. They mostly swim in the upper waters, but they will swim in all aquarium levels.
They do well on regular flake foods but thrive with live or frozen meaty foods, such as worms, crustaceans and insects. Espei rasboras (Trigonostigma espei) and Hengel’s rasboras (Trigonostigma hengeli) have a similar triangular black marking, but harlequin rasboras have a thicker, more substantial black triangle.
Breeding: After courtship, the female will rub her belly on the underside of a broad-leaved plant. The male will join her, curl his tail around the female and quiver in order to release his sperm. The breeders will do this for several hours, with the female laying six to 12 eggs each time until she lays about 100 eggs that will stick to the underside of the leaves. Because they prefer to spawn on broad leaves, provide some broad-leaved plants, such as such as Cryptocoryne and Aponogeton, in their breeding aquarium. After they have finished spawning, remove the parents from the breeding aquarium, as they may eat their eggs.
Eggs hatch in a little more than 24 hours, and the fry are free-swimming three to five days after hatching. Feed fry liquid fry foods, infusoria, rotifers, Artemia nauplii and/or newly hatched brine shrimp.
Harlequin rasboras are difficult to breed, as they need broad-leaved plants and the correct water parameters. While captive-bred specimens can do just fine in hard, alkaline water, they will not spawn or lay fertile eggs in this environment. If your goal is to breed harlequin rasboras, provide soft and acidic water (filter the water over peat), and a warm temperature (82 degrees Fahrenheit). Also feed the parents heavily with live foods before a breeding attempt.