What to Know About Harlequin Rasboras

Medically Reviewed by Vanesa Farmer, DVM on January 05, 2023
5 min read

Harlequin rasboras are common, colorful aquarium fish that carry with them a slightly deceptive name. Once thought to be a type of rasbora fish, studies showed that they were different enough that they should be put in a separate genus. Despite that, the name has stuck. Here’s everything you need to know about the harlequin rasbora fish, from biology and breeding to habitat and care.

Harlequin rasboras (Trigonostigma heteromorpha) are a species of small, colorful fish that are popular in aquariums.

When harlequin rasboras were first scientifically discovered in 1904, they were given a different scientific name — Rasbora heteromorpha. This placed them alongside other species of rasboras, small freshwater fish in the same family as carps and minnows. In 1999, after realizing their breeding behavior was closer to another type of fish, harlequin rasboras and a handful of others were placed into an adjacent genus, Trigonostigma.

There are only a few fish in the Trigonostigma genus. Most of the others are different enough that they aren’t confused for harlequin rasboras. However, a species newly discovered in 2020, Trigonostigma truncata, can be mistaken for harlequin rasboras at first glance.

Harlequin rasbora fish are shiny, and they come in shades of reddish-orange and metallic gold or silver. The term “harlequin” comes from the black triangle just below their top fin that stretches down their tails and resembles the black triangles on a harlequin costume. The fish’s fins are often tinted orange, sometimes with black edging. Their tail fins are forked, and the outer sections are often orange with the inner sections being almost transparent. The eyes of harlequin rasboras are large and round.

The harlequin rasbora’s size depends on its gender. The average length of a harlequin rasbora is 5 centimeters, but the females of the species are often larger and have rounder bodies.

Harlequin rasboras are naturally found in countries in and around the Malay peninsula in Asia: Indonesia, Malaysia, Singapore, and Thailand. They live in freshwater wetland areas such as forest pools, rivers, and streams as well as peat swamps. The decomposing plants in these areas add biomolecules called tannins into the water, creating the preferred acidic environment for harlequin rasboras.

Harlequin rasboras live in groups or schools, which tend to be larger during the day. They are happiest in groups of six or more among an abundance of plants. However, they also need an open area for swimming above and beneath vegetation, as harlequin rasboras prefer to swim near the middle of their habitat. 

The International Union for Conservation of Nature and Natural Resources (IUCN) Red List of Threatened Species lists the conservation status of harlequin rasboras “least concern,” meaning they are not considered threatened. They estimate that there are somewhere between 10,000 and 100,000 harlequin rasboras in the wild. However, most of the harlequin rasboras you can find in aquarium stores are commercially bred.

While their wild population is stable, in some areas, such as Singapore, harlequin rasboras are considered locally endangered. The largest threats to harlequin rasboras are loss of habitat due to residential, commercial, and agricultural development, and pollution due to run-off from the agriculture and forestry industries.

To breed, the female harlequin rasbora will swim upside down and rub her belly on the bottom of a broad leaf or similar structure. This is intended to catch the attention of a male. The female then lays her eggs. These eggs are sticky, and she attaches them to the underside of the leaf. The male then immediately fertilizes the eggs. Female harlequin rasboras drop about 6 to 12 eggs at a time, finally laying up to a total of 100 eggs.

This breeding method is what caused scientists to change harlequin rasboras’ genus — it’s remarkably different from the way other fish in the Rasbora genus breed. Rasbora species lay their eggs in mid-water or among plants, either on the top surface of leaves or scattering them among fine-leafed plants.

Unfortunately, harlequin rasbora parents often eat the eggs they just spawned and fertilized. If you’re trying to breed your fish, it’s recommended to separate them from the fertilized eggs to avoid this. After fertilization, the eggs have a short incubation period and usually hatch in about 18 hours. Once they’ve hatched, the larvae, which are clear, stay on the leaf for another 12 to 24 hours while they finish digesting their yolk sacs. After this, they will be able to swim freely.

In general, harlequin rasboras prefer acidic, soft water, and they will often not breed at all if the water is too alkaline or too hard. If they do spawn eggs in harder or more alkaline water, the eggs are often infertile.

Harlequin rasboras are popular aquarium fish because they’re small, pretty, and shiny. They're also known as relatively easy fish to care for. That being said, harlequin rasboras have specific needs for tank mates, tank setups, and water that must be followed if you want happy healthy fish.

Tanks and tank mates. Because harlequin rasboras live in schools in the wild, they also need to live in schools in captivity. The minimum amount of harlequin rasboras you should have in a tank is six, but they do best in groups of 10 or more. Other fish owners note that the species is compatible with similar-sized schooling fish, like other rasbora species and small tetras. Because of their mid-dwelling preference, they can also live with bottom dwellers, like nerite snails and Amano shrimps. Before mixing species of fish, however, you should consult an aquarium expert.

Harlequin fish spend more time in the midlevels of the water, and enjoy swimming in clusters. This is why a larger tank is preferable — the minimum tank volume for harlequin rasboras is 10 gallons. A longer tank works best — it should be at least 60 centimeters long. The aquarium should have a lot of plants but also plenty of open swimming space. Harlequin rasboras prefer a very gentle current and slightly dimmed lighting.

Water. Harlequin rasboras have some specific needs for their water, especially if you’re hoping to breed them. These include:

  • Acidity — Harlequin rasbora fish need an acidic water environment, ideally with a pH between 6 and 7.8. In the wild, they often swim in water with a pH of 5 to 6.5. While the harlequin rasbora can handle a pH of up to 7.5, it needs to be lower for breeding.
  • Hardness — Harlequin rasboras prefer soft water—that is, water with low concentrations of minerals. Water hardness must be less than 12 on the general hardness scale, especially for breeding. They can withstand water hardness of 2 to 15 degrees of general hardness (dGH).
  • Temperature — The harlequin rasbora is a tropical fish that needs tropical temperatures to thrive. Normally, water temperatures should be about 22 to 25 degrees Celsius. For breeding, the temperature should be warmer, ideally about 28 degrees Celsius.

Diet. In the wild, harlequin rasboras primarily eat small invertebrates — that is, creatures without a backbone. This primarily includes:

  • Crustaceans
  • Insects and insect larvae
  • Worms

In captivity, they prefer live foods, but they will also eat frozen and flake foods. The best way to ensure your harlequin rasboras’ digestive health is to feed them a mix of all three.