Dwarf Pencilfish Breeding Guide

Breeding the dwarf pencilfish (Nannostomus marginatus): sexing, the side-by-side egg-scattering courtship over fine plants, removing adults, and rearing the tiny fry.

Overview

The dwarf pencilfish, Nannostomus marginatus, is one of the smallest pencilfishes and can be spawned in densely planted, mature aquaria. It scatters its eggs over fine plant matter and provides no parental care, with egg predation rising the more individuals are involved.

Sexing

Adult males are noticeably less stocky than females. Rio Negro males additionally possess a modified anal fin in which the third to sixth rays are thickened, a useful structural distinction.

Conditioning

Condition the breeders well before spawning. Either use a densely planted mature aquarium or set up controlled containers filled with aged water for the attempt.

Breeding Setup

Include fine wool mops, Taxiphyllum or another fine-leaved aquatic plant as a spawning medium. Strong lighting and filtration are not required; an optional sponge filter is acceptable. The recorded maintenance range is 22-28 degrees C and pH 4.0-7.0, with the soft, acidic part of this range suiting spawning.

Spawning Behavior & Trigger

Courtship is structured: the male nudges the vent of the female and tries to lead her into the spawning medium. A receptive female enters with him, the pair arrange themselves side by side, and one or two eggs are released and fertilised at a time. The cycle repeats, often for several hours.

Egg & Fry Care

Remove the adults to limit egg predation, which increases with the number of fish present. The first fry become visible a day or two later. Start them on Paramecium or a proprietary dry food of 5-50 micron grade, moving on to Artemia nauplii and microworm as they grow.

Common Challenges

Because eggs are released only one or two at a time over a prolonged courtship, yields per spawning are modest, and the more adults present the more eggs are eaten. The fry are very small and require densely available micro-foods before they can take brine shrimp nauplii.