Marilyn's Pencilfish

Geographic Origin and Biotope: Amazon basin (Upper Rio Negro, Rio Vaupés, Rio Inírida). Lives in the classic "Igapó" (flooded forests) formed by extremely pure black waters and carpeted with a dense layer of decaying leaves and branches.

Taxonomy and Morphology: Marilyn's Pencilfish / Green Pencilfish (Nannostomus marilynae). Tiny pencilfish (3 cm / 1.2 inches). Visually differs from beckfordi by the much more pointed and filiform profile, almost needle-like.

Social Behavior: Very timid and gregarious (Shoaling fish). Males establish very small and temporary mobile territories among the clumps of plants, where they attract females with body vibrations.

Coloration and Sexual Dimorphism: Often labeled as "Green Pencilfish". Silver-golden background with a single defined black horizontal line running from snout to tail. Males have lime green/gold nuances along the line and a distinct red blush on the anal and lower caudal fin.

Aquarium Setup: Nano tank (min. 60 cm / 24 inches). Slow filtration with active peat and dim lighting is essential. Recreate visual barriers with floating plants (Salvinia) and thickets of Cabomba or Echinodorus.

Diet and Feeding: Mid-water micropredator. Rarely adapts immediately to dry feed. Requires brine shrimp nauplii, copepods, rotifers and microworms. If it accepts dry food, it must be reduced to a very fine powder.

Water Quality: Very sensitive. Tolerates nitrates very badly. The water must be strictly soft (GH below 8) and acidic (optimal pH 6.0-6.5) to avoid fungal infections.

Compatibility and Tankmates: Absolutely unsuitable for lively communities. To be paired exclusively with peaceful fish of its own size or smaller: Boraras, Corydoras pygmaeus or Otocinclus. Cichlids and large characins will terrify it.

Aquarium Reproduction: Very difficult (rare spontaneous reproductions documented). Scatters eggs (about 30-50 per brood) among fine mosses. Microscopic fry require paramecia and unicellular algae (green water) for the first week.

Risks and Diseases: Copper poisoning. Like many small wild blackwater characins, marilynae is lethally sensitive to the copper contained in fish medications (e.g. copper sulfate-based ich treatments); dose at half strength or use organic alternatives.