The Harlequin Rasbora is one of the most commonly encountered rasboras, and for good reason. It has a beautiful pattern of gold and red, set off by a blueish black wedge. A small, shoaling fish, Harlequins should never be kept in groups of less than a half dozen, and would appreciate being kept in at least a dozen.
There are a handful of other members of the genus Trigonostigma which make it into the hobby, all of which are superficially similar. The Harlequin has the most orange, and the largest black wedge. The most similar species, which is often sold under the same name, is T. espei, which has a much more narrow wedge. We prefer to call this fish the "Lambchop Rasbora." In addition to these similar cogeners, there is also an all black Harlequin, in which the black wedge covers most of the body, with little or no orange on the fish.
Harlequins make excellent additions to planted tanks, and will generally leave shrimp and other small inverts alone. They can also be used as dither fish for many small, shy Dwarf Cichlids and Anabantoids. Hailing from Southeast Asia, they share the same waters as many of the Betta species, and mixing them poses little problem, provided you aren't interested in breeding the Betta (as rasboras will eat the fry).