Harlequin Rasbora (Trigonostigma heteromorpha)

The Harlequin Rasbora (Trigonostigma heteromorpha) is the fish that made rasboras famous: a copper-pink body carrying one bold black wedge, moving as a tight group through the middle of the tank. It has been a community-tank staple for a century for good reason, hardy, genuinely peaceful, small enough for a modest tank, and striking in a way that photos never quite catch. Put ten of them over a dark substrate in a planted tank and they read like a single moving shape. Browse the rest of our freshwater livestock to build the community around them.

Care Guide

Harlequins come from gently flowing forest streams and peat swamps in Thailand, Malaysia, Singapore, Sumatra and Borneo, where the water runs soft, tannin-stained and thick with Cryptocoryne. They are adaptable enough for a normal community tank, but they show their best color when the tank leans toward that: dim or plant-shaded light, dark substrate, and a few catappa leaves in the water.

• Group size: eight to ten minimum. This is the single biggest thing people get wrong. A group of three sits in a corner and stays washed out; a real school spreads out, holds formation and colors up.
• Tank size: a 24 x 12 inch footprint (about 15 gallons) is the practical minimum, and a 20 gallon long is a better home for a school of ten.
• Water: 72 to 79°F, pH 6.0 to 7.5, soft to moderately hard. They tolerate a wider band than that (published ranges run 70 to 82°F and down to pH 5.0), but stability matters more than hitting a number.
• Hardness is the local catch: our Lawrenceville tap is extremely soft, below the 1 to 12 dGH range this fish wants. If you keep them on straight tap, a remineralizer on every water change keeps the general hardness where it belongs.
• Diet: a micropredator in the wild. Quality flakes and small granules as the staple, plus frozen or live daphnia, brine shrimp and bloodworms a couple of times a week for color and conditioning.
• Behavior: very peaceful, active, mid-water. They do not nip and they do not bother anything.
• Lifespan: 5 to 6 years with good care, which is long for a fish this size.

Specifications

Compatibility

Harlequins are one of the easiest community fish to plan around. They stay in the middle of the tank, they ignore everyone, and the only real rule is keeping them away from anything big enough to treat a two inch fish as food.

• Good tankmates: small tetras and other rasboras, Cherry Barbs, Zebra Danios, and bottom dwellers like Panda Corydoras and the Kuhli Loach.
• Also great with: peaceful gouramis such as the Pearl Gourami, nerite snails and shrimp, and a heavily planted tank, see our live plants.
• Use caution with: anything large or predatory, including angelfish once they are grown, and boisterous fish that will outcompete them at feeding time.
• Keep the school intact. Numbers are the whole trick with this fish. Eight is the floor, twelve is where it starts to look like the pictures.

Recommended Foods & Supplies

• Quality tropical flakes or small granules as the daily staple, their mouths are small so size the food accordingly
• Frozen daphnia, brine shrimp and bloodworms two or three times a week for color
• Catappa leaves for the tannin-stained water they evolved in
• Live plants and a dark substrate, the fastest way to deepen their copper color

FAQ

How many should I buy?
Eight at an absolute minimum, ten to twelve if the tank allows. This is not an upsell, it is the difference between a shy fish hiding in a corner and the schooling display you bought them for.

Are they good for beginners?
Yes, one of the best. They are hardy, adaptable, long lived and completely peaceful.

How do I tell males from females?
Look at the black wedge. On males the lower edge is sharp and well defined; on females it is rounder and softer. Females are also a little larger and fuller bodied.

Will they work in a 10 gallon?
We would not recommend it. A proper school needs a 24 inch footprint, so a 15 or 20 gallon long is the realistic starting point.

Do they need special water?
Not special, but do check your hardness. Local tap here is very soft and sits under their preferred range, so a remineralizer added with each water change keeps them in good shape long term.