Trigonostigma espei

Native to the blackwater streams and peat swamps of Southeast Asia, the Lambchop Rasbora is a species adapted to environments rich in tannins and organic matter. These habitats are characterized by soft, acidic water stained to a tea-like color by decaying leaves, with dense aquatic vegetation and a gentle current. To best replicate this natural setting in an aquarium, a heavily planted tank is recommended, offering the security and shaded areas this fish prefers. The inclusion of driftwood, dried botanicals like catappa leaves, or peat in the filter can help create the desired blackwater conditions, which not only mimics their environment but can also bolster their immune systems and encourage natural behaviors.

This species is an obligate shoaler, and its well-being is intrinsically linked to being part of a group. Keeping them in appropriately large numbers is critical for reducing stress and allowing their fascinating social structure to emerge. In a group, they become more confident, active, and display their best coloration and behaviors. While they appreciate dense plant cover, they are also active swimmers that require open water space in the middle and upper strata of the aquarium. Therefore, a balance of planted zones and open swimming areas is ideal.

As an omnivore, their wild diet consists mainly of small insects, crustaceans, and zooplankton. In captivity, they require a varied diet to ensure proper nutrition. A high-quality micro-pellet or flake food can serve as the staple, but this should be supplemented regularly with frozen or live foods such as daphnia, brine shrimp, and grindal worms. This variety not only promotes optimal health and vitality but also stimulates their natural foraging instincts. They should be fed small amounts daily. Due to their peaceful nature and small adult size, they are excellent candidates for a community aquarium, provided their tank mates are of a similar size and temperament. Good companions include other small rasboras, tetras, peaceful bottom-dwellers like Corydoras catfish, and dwarf gouramis.

Trigonostigma espei belongs to the family Danionidae, a large and diverse group of cypriniform fishes. The genus Trigonostigma is distinguished by its members' characteristic triangular or hatchet-shaped body marking, and T. espei is differentiated from its congeners by the specific morphology of this marking. The species name, espei, is a patronym honoring the German aquarist Heinrich Espe, who was instrumental in its introduction to the aquarium hobby. Morphologically, T. espei exhibits a fusiform body shape, which is compressed laterally and streamlined for efficient locomotion through the water column. This body plan is typical for active, shoaling species that inhabit the mid-to-upper levels of their environment.

Ecologically, T. espei functions as a micropredator in its native Southeast Asian blackwater habitats. It primarily forages on small invertebrates, zooplankton, and insect larvae, playing a role in regulating these microscopic populations. In turn, it serves as prey for larger piscivorous fish and wading birds, forming an important link in the local food web. Its adaptation to soft, acidic waters with a high concentration of dissolved tannins from decaying vegetation is a key physiological trait. This species demonstrates a tolerance for a range of acidic pH levels and very low water hardness, which is characteristic of fish evolved in such environments. Its metabolism is relatively low, resulting in low oxygen consumption and minimal waste production, an advantageous trait in slow-moving water bodies where oxygen levels may fluctuate. Its current IUCN Red List status as 'Least Concern' (LC) suggests that the species is widespread and not facing immediate, significant threats to its survival on a global scale, although local populations may be vulnerable to habitat loss and pollution.

Breeding Trigonostigma espei is considered moderately difficult and presents a rewarding challenge for the dedicated hobbyist. Success largely depends on replicating the specific water conditions that trigger spawning. For this purpose, a separate, dedicated breeding aquarium is highly recommended. This tank should be equipped with very soft and acidic water, which can be achieved through the use of reverse osmosis water remineralized to a low hardness, along with peat filtration or the addition of catappa leaves to lower the pH and release beneficial tannins. Lighting should be kept very dim, as both the adult fish and the eggs are sensitive to bright light.

Sexual dimorphism is subtle. Females are typically larger and have a fuller, more rounded abdomen, especially when they are conditioned and carrying eggs. Males are generally slimmer, more streamlined, and may exhibit more intense coloration when in spawning condition. For a successful attempt, it is best to condition a group of males and females separately on a rich diet of live and frozen foods. The ideal ratio in the breeding tank is often two males for every one female to increase the chances of fertilization.

This species is an egg-scatterer that displays a unique spawning behavior. They will deposit their adhesive eggs on the underside of broad-leafed plants, such as Cryptocoryne or Anubias species. During their courtship ritual, the pair or trio will swim together until they turn upside-down beneath a suitable leaf surface, where the female releases a few eggs that are immediately fertilized by the male(s). This process is repeated until the female has expelled all her eggs. Once spawning is complete, the adult fish must be removed promptly from the tank, as they have strong parental-culling instincts and will consume their own eggs. The eggs typically hatch within 24-48 hours. The fry will subsist on their yolk sac for another few days before becoming free-swimming. At this stage, they are incredibly small and must be fed with microscopic foods like infusoria or liquid fry food. As they grow over the next week, their diet can be transitioned to newly hatched brine shrimp and micro-worms.