Nannostomus marginatus
The smallest pencilfish. Three horizontal black-and-red stripes against a pale gold body. Holds itself horizontally, hovering mid-water rather than swimming actively. Loves blackwater conditions and dense plant cover. Goes pale at night, a distinctive 'sleep colour' that surprises new keepers.
Dwarf Pencilfish (Nannostomus marginatus) reaches 3–3.5 cm as an adult and needs a minimum tank of 60 L. Native to Western Guyana, Suriname, lower Amazon basin, it lives in the mid to top water column with a peaceful temperament. Aim for 22–28 °C, pH 5.0–7.0, and 1–8 dGH hardness. Lifespan is 3–5 years with good care. Keep dwarf pencilfish in groups of 6+, yes (loose shoal) schoolers need numbers to display natural behaviour. Diet: micropredator, Micro pellets, baby brine shrimp, daphnia, microworms. Tiny upturned mouth, feed small floating or slow-sinking foods. Plant-safe: Yes. Shrimp-safe: Yes (adult shrimp).
Care at a glance
The smallest pencilfish. Three horizontal black-and-red stripes against a pale gold body. Holds itself horizontally, hovering mid-water rather than swimming actively. Loves blackwater conditions and dense plant cover. Goes pale at night, a distinctive 'sleep colour' that surprises new keepers.
By Mike ElmiraUpdated 2 min read
Part of our complete guide to aquarium fish for the planted tank.
The parameters that decide whether dwarf pencilfish fits in your tank.
Mid to Top
Yes
Group of 6+
Lebiasinidae
Micropredator
Micro pellets, baby brine shrimp, daphnia, microworms. Tiny upturned mouth, feed small floating or slow-sinking foods.
3–5 yrs
Medium
Tiny blackwater rivulets of the upper Rio Negro
Western Guyana, Suriname, lower Amazon basin
Tank-mate safety and the species this one is documented to thrive (or fail) alongside.
Best species-only or with the smallest community species: chili rasboras, ember tetras, sparkling gouramis, cherry shrimp adults.
Anything larger than ~4 cm — pencilfish will hide and refuse to display. Fin-nippers.
Hard-won lessons from the tank.
Watch them at night — they shift from their daytime striping to a paler 'sleep colour' that can alarm new keepers. This is normal behaviour. A small group of 8+ in a blackwater nano with Java moss and floating plants is an elite specialty display rarely seen in chain-store aquariums.
Genus 'Nannostomus' = 'small mouth'. Species 'marginatus' = 'edged/bordered' (referring to the marked stripes).
What can go wrong and how to spot it.
Failure modes, in order of how dramatic the fix is.
Sensitive to alkaline water (rare in trade outlets that don't maintain blackwater conditions). Susceptible to wasting in poor-quality QT.
Often labelled as 'easy nano fish' — they're easy ONLY in proper soft-water blackwater. Standard community-tank pH 7.5+ kills them slowly.
The practical routine, read top to bottom.
60 L+ nano blackwater. RO + tannins for best health. Heavily planted with mid-water cover (java fern, anubias, mosses). Surface plants for security. Sponge filter only.
3 weeks. Provide tannins and dim cover. Watch for slow wasting common in stressed pencilfish.
Where it comes from, how it behaves, and the variants you'll see at retail.
Where it lived before it came home.
Slow blackwater tributaries of the lower Amazon basin. Tannin-stained, very soft, acidic water with dense leaf litter and submerged vegetation.
Tiny insect larvae, micro-crustaceans, falling terrestrial invertebrates. Strict micropredator.
Not threatened. Mostly wild-caught from sustainable fisheries.
How they pair, reproduce, and grow.
Males slimmer with intense red colour on the lower stripes. Females rounder with paler colour and a more visible belly profile.
Egg scatterers among Java moss. Soft very acidic water at 26 °C, very dim lighting. Adults eat eggs. Fry are tiny — infusoria required for first 10 days.
The named cultivars and the lookalikes worth flagging.
Wild type only. N. trifasciatus (three-lined pencilfish), N. eques (diptail pencilfish), N. beckfordi (golden pencilfish) are commonly sold and sometimes confused. Marginatus has three horizontal black stripes and a small adult size.
Direct answers to the questions search engines and AI assistants surface most often about this species.
Dwarf Pencilfish (Nannostomus marginatus) needs a minimum tank of 60 L. They live in the mid to top water column and should be kept in groups of 6+, so a longer footprint matters more than depth.
Target 22–28 °C, pH 5.0–7.0, and 1–8 dGH hardness. Acclimate slowly when moving them between water sources.
Shrimp safety: Yes (adult shrimp). Plant safety: Yes.
Dwarf Pencilfish are micropredator. Micro pellets, baby brine shrimp, daphnia, microworms. Tiny upturned mouth, feed small floating or slow-sinking foods.
On Fin & Stem's 1–5 difficulty scale this species rates 3/5. Intermediate, stable parameters and a mature tank matter. Breeding difficulty: medium.
Typical lifespan in a well-maintained tank is 3–5 years.
A planted tank is a system. Pair this fish with one entry from each other pillar to plan the whole scape.
