Marilyn’s Pencilfish (Nannostomus marylinae)

Species Profile:

• Origin: This species is native to the upper Amazon Basin, specifically the Rio Negro and Rio Orinoco systems in Brazil and Colombia.
• Environment: They inhabit slow-moving “blackwater” tributaries and swampy areas. These habitats are typically enriched with humic acids from decaying organic matter, resulting in very soft, acidic water with a tea-coloured tint.

• Size: A diminutive species, rarely exceeding 3.0–3.5 cm in total length.
• Appearance: The body is exceptionally slender. A prominent dark horizontal stripe runs from the snout to the caudal fin, topped by a thinner, shimmering emerald-to-gold line.
• Diurnal Rhythm: Like other Pencilfish, N. marylinae undergoes a dramatic colour change at night. Their horizontal stripes fade, and two to three dark vertical bars appear as a form of nocturnal camouflage.
• Swimming Posture: Unlike the “One-lined Pencilfish” (N. unifasciatus), which swims at a 45-degree angle, Marilyn’s Pencilfish typically swims in a horizontal position.

• Tank Choice: Due to their size, they are perfect for the smaller fish tank 40-100ltrs.
• Water Chemistry:
  • Temperature: 23°C – 28°C.
  • pH: 5.0 – 7.0 (Strictly acidic to neutral).
  • Hardness: 1 – 10 dGH (Very soft water is preferred).
• Aquascape: A heavily planted environment is essential. Use fine-leaved plants like Cabomba or Mayaca fluviatilis and incorporate floating plants (such as Salvinia) to provide the shaded areas where they feel most secure.

• Micro-Predator: In the wild, they feed on tiny invertebrates and zooplankton.
• Aquarium Diet: They have extremely small mouths. Provide finely crushed flakes, micro-granules, and live or frozen foods such as Cyclops, Moina, or freshly hatched Brine Shrimp.
• Note: They are slow feeders and may be outcompeted by boisterous tetras.

• Social Needs: They are shoaling fish and should be kept in a group of at least 8–10 individuals. In smaller numbers, they become excessively timid.
• Tankmates: Ideal companions include other small, peaceful inhabitants such as Hoplisoma sp. CW217, Boraras rasboras, and various dwarf shrimp species.
• Aggression: Males may engage in “mock fighting,” where they display side-by-side. This is rarely harmful and serves to establish a social hierarchy.

• Lid Security: This species is a proficient jumper. Ensure your aquarium has a tight-fitting lid, like the glass cover provided with fish tanks.
• Water Stability: They are sensitive to fluctuating water parameters. Regular small water changes (10-15% weekly) are better than large, infrequent ones.