Nannostomus marginatus

This small, peaceful shoaling fish from South America is an excellent choice for community aquariums, known for its unique horizontal swimming posture. It thrives in groups and should be kept with several of its own kind to feel secure and display its natural behaviors. In the aquarium, it prefers to swim in the top and middle water levels. An ideal environment for this fish includes calm water with low flow, mimicking the gentle streams of its native habitat. As an omnivore, it is not a picky eater and will readily accept a varied diet of high-quality flakes, small pellets, and occasional treats of frozen or live foods, which should be offered daily.

Due to its very small size and peaceful nature, it can be easily intimidated by tank mates that are very large or overly active. It is best housed with other small, gentle species. While generally easy to care for, it has specific water parameter needs, making it a moderately challenging but rewarding fish for a properly maintained aquarium. With good care, it can live for several years, adding activity and interest to the upper portions of your tank.

A delightful shoaling species originating from the slow-moving waters of South America, this fish brings a unique presence to a well-planned aquarium. Its natural habitats are often characterized by soft, acidic water, dense aquatic vegetation, and submerged wood, creating a complex environment. Replicating these conditions is key to its long-term health. A planted aquarium with plenty of driftwood and leaf litter will not only provide necessary cover, making the fish feel secure, but will also encourage the growth of biofilm, which this species enjoys grazing on as a natural part of its diet.

Maintaining stable water parameters within its preferred warm, soft, and acidic-to-neutral range is crucial. Water movement should be kept to a minimum, as it is not adapted for strong currents and may become stressed. Its social structure is built around shoaling, and it must be kept in a group. A larger school will promote natural behaviors, reduce stress, and diffuse minor territorial disputes that can occur between males. These disputes are typically harmless displays rather than aggressive attacks. When choosing tank mates, select other small, peaceful species that won't outcompete it for food or cause stress with hyperactive behavior. Large or boisterous fish are unsuitable companions.

As an omnivore with a small mouth, its diet should be varied and appropriately sized. A high-quality micro-pellet or crushed flake can serve as a staple, but this should be supplemented regularly with live or frozen foods to ensure optimal nutrition. Feeding should occur once daily. This species has a relatively low metabolism and produces minimal waste, making it a low-bioload fish. However, consistent maintenance and water changes are still essential for its well-being, as it can be sensitive to deteriorating water quality. Under ideal conditions, it can live for a number of years.

Belonging to the family Lebiasinidae, this species is a classic example of the fusiform body shape common to many members of its order, which is an adaptation for agile movement. As a member of its genus, it exhibits a small, superior mouth, indicative of a diet that includes surface-dwelling invertebrates and biofilm grazed from submerged structures. Its ecological niche in its native South American waterways is that of a micro-predator and primary consumer, occupying the upper and middle strata of the water column in low-flow environments like minor tributaries and backwaters.

Its physiology is well-suited to these habitats, demonstrating low oxygen consumption and low waste production, which are advantageous traits in warm, slow-moving, and sometimes oxygen-depleted waters. Behaviorally, it is a shoaling species, a strategy primarily for predator avoidance. Intra-species territoriality is observed, particularly in males, and is likely linked to competition for resources and breeding opportunities. The observed stress response to large or highly active cohabitants is a direct manifestation of its anti-predator instincts, where such stimuli are perceived as threats. Its swimming style is characterized by periods of hovering punctuated by quick, darting movements, which serve both for capturing prey and for social interactions.

Its environmental interactions include acting as a biofilm consumer, contributing to the control of aufwuchs on submerged surfaces. This foraging behavior highlights its role in the nutrient cycling of its ecosystem. The IUCN has not evaluated the conservation status of this species, indicating a need for further research into its wild populations and potential threats.

Successfully breeding this species is considered a moderately difficult endeavor that requires a dedicated approach. To begin, a separate breeding aquarium is highly recommended. This tank should be equipped with a gentle sponge filter to ensure water cleanliness without creating strong currents that could harm delicate fry. The water should be very soft, acidic, and maintained at the higher end of the species' preferred temperature range to help induce spawning. The inclusion of fine-leaved plants, such as java moss, or synthetic spawning mops is essential, as these provide the necessary surfaces for egg deposition.

A group of well-conditioned adults should be introduced into the breeding tank, ideally with a higher ratio of females to males to reduce male aggression and increase the chances of a successful spawn. Conditioning involves feeding the fish a rich diet of live and frozen foods for several weeks prior to the breeding attempt to bring them into prime reproductive health. This species is an egg-scatterer, meaning the female will release adhesive eggs among the provided plants or mops, which are then fertilized by the male. They exhibit no parental care and will readily consume their own eggs if given the chance. Therefore, it is critical to remove the adult fish as soon as spawning is complete.

The eggs typically hatch within a few days. The resulting fry are extremely small and will first nourish themselves from their yolk sacs. Once they become free-swimming, they require microscopic foods. A culture of infusoria is an excellent initial food source. As the fry grow, they can be transitioned to slightly larger foods like microworms and newly hatched brine shrimp. Meticulous attention to water quality is paramount for fry survival; small, frequent water changes are necessary to keep the water pristine without causing undue stress. The fry are exceptionally sensitive to any fluctuations in their environment.