Red Arc Pencilfish Care Guide: Vivid Stripes That Hover

This red arc pencilfish care guide details everything needed to keep Nannostomus mortenthaleri, one of the most visually striking pencilfish species, in a planted aquarium. At Gensou Aquascaping, found at 5 Everton Park, Singapore, we admire this fish for its bold crimson stripes and unique hovering swimming style that adds a distinctive character to any aquascape.

Species Overview

The red arc pencilfish is native to blackwater tributaries of the Rio Nanay in Peru, where it lives among submerged roots and leaf litter in slow-moving, tannin-stained waters. Adults reach approximately 3 cm in length and display three vivid red horizontal stripes separated by bands of black and cream.

Unlike many small characins, pencilfish hold themselves at a slight upward angle and move with deliberate, hovering motions. This behaviour, combined with their intense colouration, makes them a captivating addition to well-planted setups. They are occasionally available in the Singapore trade, though less common than some other nano species.

Tank Size and Aquascape Design

A 40-litre aquarium is suitable for a small group of six to eight red arc pencilfish. A 60-litre tank provides more room for a larger shoal and allows for a richer aquascape. These fish appreciate dense planting, particularly with fine-leaved stems and mosses that replicate the vegetation along their native riverbanks.

Driftwood branches and dried leaves such as catappa or guava create a naturalistic blackwater feel while releasing tannins that the fish find comforting. Keep the lighting moderate to subdued; overly bright conditions can wash out their colouration and increase stress. A dark substrate accentuates their red and black pattern beautifully.

Water Parameters

Red arc pencilfish thrive in soft, acidic water. Target a temperature of 24 to 28 degrees Celsius, which aligns well with Singapore’s ambient conditions and typically requires no heater in non-air-conditioned rooms. Aim for a pH of 5.0 to 6.5 and general hardness below 5 dGH for the best colour display.

Achieving these soft, acidic conditions in Singapore may require an active buffering substrate or the use of reverse osmosis water blended with remineraliser. Peat filtration or botanical extracts also help lower pH naturally. Weekly water changes of 15 to 20 per cent keep the environment fresh without disrupting the stable chemistry these fish need.

Diet and Feeding

Red arc pencilfish are micro predators that feed on tiny invertebrates in the wild. In captivity, they accept high-quality micro pellets and crushed flakes, though they truly thrive on a diet supplemented with live and frozen foods. Baby brine shrimp, daphnia, cyclops, and grindal worms are all readily taken.

Feed small portions two to three times daily. Pencilfish tend to eat slowly, picking at individual food particles rather than gulping. Ensure that food remains suspended in the water column long enough for them to consume it, especially if housed with faster-feeding tank mates.

Behaviour and Social Dynamics

Males are territorial and will display intensely to each other, spreading fins and deepening their red coloration during sparring. These confrontations are largely ritualistic and rarely cause injury. Keeping a ratio of one male to two or three females helps distribute aggression and encourages the most vibrant display.

Red arc pencilfish occupy the middle to upper water column, hovering among plant stems and hardscape. Their calm, almost meditative movement contrasts nicely with the busier activity of other nano species, adding a layer of visual interest to community setups.

Choosing Tank Mates

Select companions that share a preference for soft, acidic water and a peaceful disposition. Cardinal tetras, ember tetras, chocolate gouramis, pygmy corydoras, and small Apistogramma species make excellent choices. Caridina shrimp thrive in similar water parameters and are generally safe alongside pencilfish.

Avoid boisterous or large species that could intimidate these relatively shy fish. Fast-moving barbs or aggressive cichlids are not appropriate. In a biotope-style setup replicating a South American blackwater stream, red arc pencilfish are the star attraction.

Health and Quarantine

These fish can be delicate upon arrival, particularly wild-caught specimens. A quarantine period of two to three weeks is strongly recommended. Drip acclimate new arrivals carefully to avoid osmotic shock, especially when transitioning from the soft water of exporters to local conditions.

Common ailments include bacterial infections and external parasites. Maintain stable water parameters and excellent filtration to minimise disease risk. In Singapore’s warm environment, ensure the quarantine tank has adequate aeration and avoid overcrowding during the observation period.

Why Red Arc Pencilfish Deserve a Place in Your Aquascape

The red arc pencilfish offers a unique combination of bold colour and serene behaviour that few other nano fish can match. Their hovering habit creates a sense of calm elegance in any planted layout. For Singapore aquascapers seeking something distinctive beyond the usual nano selections, this species is well worth the effort. Consult the team at Gensou Aquascaping, 5 Everton Park, for sourcing tips and setup advice tailored to local conditions.

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