Marginatus Pencilfish

Nannostomus marginatus parvulus — often simply called the Marginatus or Dwarf Pencilfish variant — is a diminutive form found in blackwater drainages of the Amazon and Orinoco systems. Its colouration is subtler than many relatives: a warm brownish-gold background with a diffuse dark lateral band and faint orange-red fin flushes that emerge fully only in ultra-soft, acidic water under optimal conditions.

This fish is best suited to a dedicated species tank or a carefully curated blackwater biotope. It is sensitive to water quality changes and requires very low nitrates, a stable pH below 6.5, and the darkening effect of tannins to truly thrive. Despite its delicate requirements, it rewards patient aquarists with complex social interactions within a group.

Feeding on the smallest available live and frozen foods is essential — rotifers, baby brine shrimp nauplii, micro-worms, and cyclops. Commercial micro-pellets of the finest grade may be accepted by established fish.

Water: 23–27°C, pH 4.5–6.5, extremely soft (GH 0–5, KH 0–2); RO water blending advised. Tank: 40 L minimum; blackwater setup, botanicals, peat filtration, very dim lighting. Feeding: Rotifers, baby brine shrimp nauplii, micro-worms, cyclops; micro-pellets occasionally accepted. Breeding: Egg-scatterer; very soft acidic water essential; fry require rotifers and infusoria. Compatibility: Species tank or blackwater biotope only — other equally small and delicate species.