How to Breed Harlequin Rasboras: Under-Leaf Spawners

Harlequin rasboras (Trigonostigma heteromorpha) breed in a way almost no other common aquarium fish does — the female flips upside down and deposits adhesive eggs on the underside of a broad leaf. Witnessing this behaviour for the first time is unforgettable. This breed harlequin rasbora guide from Gensou Aquascaping at 5 Everton Park, Singapore, covers the soft-water setup, spawning triggers and fry-rearing techniques you need to succeed.

Why Harlequins Are Worth Breeding

Despite being one of the most widely sold tropical fish, harlequin rasboras are rarely bred by hobbyists. Most stock comes from commercial farms in Southeast Asia, including right here in the region. Breeding them at home gives you control over genetics and lets you observe a unique reproductive strategy. Captive-bred fish also tend to be hardier and adapt better to local water conditions than wild-caught imports that have endured long shipping chains.

Selecting Breeders

Pick adults between 8 and 14 months old from a healthy, active school. Males are slimmer and display a more pointed, angular black triangular patch on their flank. Females are rounder, especially when full of eggs, and their triangle has a straighter lower edge. Select two males for every female to ensure good fertilisation. Condition them separately for 10-14 days on frozen daphnia, cyclops and live micro worms.

The Spawning Tank

Set up a 20-30 litre tank with a thin layer of dark substrate or bare bottom. The essential addition is broad-leaved plants — Cryptocoryne wendtii, Echinodorus species or even plastic broad leaves angled at roughly 45 degrees. Harlequins deposit eggs on the underside of these leaves, so the leaves must be sturdy and positioned 10-15 cm below the surface. An air-driven sponge filter provides gentle circulation. Dim the lighting — a single low-wattage LED or ambient room light is sufficient.

Water Parameters

Soft, acidic water is essential for egg viability. Target pH 5.5-6.5, GH 1-3 and temperature of 26-28 °C. Singapore’s tap water at GH 2-4 is close but benefits from mixing with RO water at a 1:1 ratio to push hardness lower. Acidify with peat filtration or Indian almond leaves — three to four dried leaves in the tank for a week before spawning stains the water a light amber and brings the pH down naturally. The tannins also have mild antibacterial properties that protect the eggs.

Triggering Spawning

Introduce the conditioned group in the evening after a 20% water change with slightly cooler water (2 °C below tank temperature) to simulate a rain event. Spawning usually begins the following morning. The male courts the female with fluttering displays beneath a chosen leaf. When receptive, she flips belly-up against the leaf’s underside and deposits a small cluster of eggs while the male wraps alongside to fertilise them. The process repeats across several leaves over one to two hours, yielding 80-150 eggs total.

Egg Care and Hatching

Remove the adults once spawning concludes to prevent egg predation. Eggs are small, amber-tinted and stuck firmly to the leaves. They hatch in 24-28 hours at 27 °C. Add a single drop of methylene blue per 10 litres as antifungal protection. Keep lighting very low during incubation. The fry hang vertically from the leaf surface for another 24 hours before becoming free-swimming, at which point they drop down and begin hunting for microscopic food.

Raising the Fry

First food is infusoria or paramecium for five to seven days. Transition to freshly hatched baby brine shrimp once the fry are large enough to chase them — usually around day seven to ten. Feed small amounts three times daily and siphon uneaten food with airline tubing. Perform 10% daily water changes with matched soft water. Growth is moderate; by six weeks the juvenile shape and the beginnings of the trademark triangle become visible. At three months, young harlequins are ready for a community tank.

Troubleshooting Low Hatch Rates

If eggs turn white within 12 hours, they were likely unfertilised — this usually means the water was too hard or the male was not conditioned well enough. Fungus spreading from dead eggs to viable ones is the second most common problem; methylene blue and prompt removal of opaque eggs with a pipette solve this. Finally, ensure the spawning leaves are clean and free of algae, which can smother eggs. With practice and the right water, harlequin breeding becomes repeatable. Gensou Aquascaping stocks Indian almond leaves and sponge filters ideal for soft-water breeding setups.

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