Nannostomus mortenthaleri

Mortenthaler's Pencilfish, Nannostomus mortenthaleri, is a demanding but rewarding species for the dedicated aquarist. Its natural habitat in South America consists of slow-moving blackwater streams and tributaries. To replicate this, an aquarium with very low water flow is essential. The environment should be rich in tannins, with soft, highly acidic water, which can be achieved through the use of botanicals like leaf litter and driftwood. These additions not only help buffer the water to the required parameters but also provide surfaces for biofilm to grow, which this fish consumes as a natural part of its diet.

A mature aquarium is crucial, as these fish are sensitive to fluctuations in water quality. A well-established nitrogen cycle and stable parameters are non-negotiable. Aquascaping with dense plantings, including floating varieties, will provide security, diffuse lighting, and create distinct territories, which is important for managing their social dynamics. Although a shoaling species, males are known to be territorial and will engage in sparring displays to establish a hierarchy. Keeping them in a sufficiently large group in a tank with adequate space and visual barriers helps to disperse this aggression, ensuring no single individual is overly stressed. They primarily occupy the top and middle water columns. Their omnivorous diet should be varied and consist of high-quality, small-sized foods. Crushed flakes and micro-pellets can form the staple, but this should be supplemented frequently with live and frozen foods to promote optimal health and vitality. Due to its specific needs for pristine, soft, and acidic water, this species is considered difficult to keep and is not recommended for beginners.

Nannostomus mortenthaleri is a small freshwater fish belonging to the family Lebiasinidae, commonly known as the pencilfishes. As a member of the order Characiformes, it shares characteristics with other characins but is distinguished by the unique traits of its family and genus. Its morphology is described as fusiform, a spindle-like body shape that is hydrodynamically efficient for its moderate activity level within the water column. This species is native to specific drainages in South America, where it inhabits lentic, blackwater environments characterized by extremely low mineral content, a highly acidic pH, and an abundance of dissolved tannins from decaying organic matter.

Physiologically, N. mortenthaleri is adapted to its niche habitat. It exhibits low oxygen consumption and subsequently low waste production, resulting in a minimal bioload factor. This adaptation is advantageous in slow-moving, vegetation-dense waters where dissolved oxygen levels can be lower. Its diet as an omnivore includes a significant role as a biofilm consumer, indicating it actively grazes on aufwuchs—the collection of small animals, plants, and microorganisms attached to submerged surfaces. This feeding behavior classifies it as a micro-predator within its ecosystem. Socially, it is a shoaling species that exhibits intraspecific territoriality, a common behavior in the genus Nannostomus, where individuals, particularly males, compete for dominance and resources. Currently, its conservation status is listed as Not Evaluated by the IUCN, signifying a lack of formal assessment of its population in the wild.

Captive propagation of this species is widely regarded as difficult and presents a significant challenge even for experienced aquarists. Success requires a meticulously controlled, dedicated breeding aquarium separate from the main display. This tank must accurately replicate the species' natural spawning conditions: exceptionally soft and acidic water, minimal water flow, and very dim lighting. The inclusion of fine-leaved plants, such as mosses, or synthetic spawning mops is essential to provide a medium for the fish to deposit their eggs.

To induce spawning, a conditioned group of adults, ideally with more females than males, should be selected. Conditioning involves a diet rich in live and frozen foods for several weeks to encourage egg development in females and milt production in males. Distinguishing between sexes can be subtle, but typically, well-conditioned males are more intensely colored and slimmer than the rounder, egg-laden females. This species reproduces via egg-scattering, where the female releases a small number of adhesive eggs at a time, which are then fertilized by the male and stick to the provided spawning media. Spawning events usually occur in the early morning hours. Adults exhibit no parental care and will actively predate their own eggs; therefore, it is critical to remove the parents immediately after spawning is complete. The eggs are highly susceptible to fungus and sensitive to light, so the tank should be kept dark and impeccably clean. After hatching, the minuscule fry will first consume their yolk sac before requiring microscopic foods such as infusoria. As they grow, their diet can be gradually shifted to larger live foods like newly hatched brine shrimp. The entire process, from egg to juvenile, demands pristine water quality and careful attention to feeding.