Myloplus rubripinnis

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The Red Hook Silver Dollar, sold under the name Myloplus rubripinnis, is a South American characin with a deep, silvery body, valued for its red anal fin that contrasts with its metallic coloration. This schooling species prefers warm, well-oxygenated water and plenty of swimming space. It is best suited to large community aquariums containing robust, peaceful fish. As with several Amazonian fishes offered in the aquarium trade, some specimens may be collected from the wild; careful acclimation and a varied diet are therefore essential. With a rarity rating of 1 out of 3, it remains fairly accessible, although it is not available in every aquarium store.

General species description

Myloplus rubripinnis is a freshwater fish from the family Serrasalmidae, related to silver dollars and piranhas. Despite this relationship, it does not show the typical predatory behavior of piranhas and mainly feeds on plant matter, fruit and small invertebrates. Its laterally compressed body and open-water swimming style resemble those of other silver dollars. It is a schooling fish that does best in a group, ideally in a long aquarium with an unobstructed swimming area. Adults can become fairly large, so a spacious tank is necessary for long-term care. Wild-collected specimens are often more sensitive to sudden changes in water conditions and benefit from quarantine, gradual acclimation and a structured environment with retreat areas. In the trade, this species is less common than some Metynnis, but it remains sought after for its red fins.

Geographic origin and habitat

Reference country: Brazil, with a South American distribution associated with the Amazon Basin. Myloplus rubripinnis inhabits tropical waterways, quieter river sections and areas influenced by riverside vegetation. During seasonal flooding, water may cover wooded margins, roots and terrestrial plants, creating productive feeding areas. The water may be clear, tea-colored or lightly stained by organic matter. Its natural environment generally provides warm, soft to moderately acidic water, with branches, roots, leaf litter and submerged vegetation. In the aquarium, it is more important to reproduce swimming space, thermal stability and side shelters than to build an overloaded miniature biotope. Commercial populations may include wild-collected fish; efficient filtration and strong oxygenation help reduce acclimation stress.

Precise location and local name

The distribution of Myloplus rubripinnis is generally associated with the Amazon Basin, especially river systems in Brazil and neighboring Amazonian regions. Trade records sometimes provide a more precise collection locality, but these localities vary between shipments and should not be generalized to the entire species. No single local name is consistently used in the international aquarium trade; the English name Red Hook Silver Dollar is the most common among retailers and aquarists. Silver dollar refers to the fish’s rounded, flattened and reflective body, while hook describes the shape and coloration of the anal fin. For wild-collected specimens, the exact locality should be preserved by the supplier whenever it is available, since transport, acclimation and maintenance conditions may differ among collection areas.

Appearance and physical characteristics

The body of Myloplus rubripinnis is deep, strongly compressed and covered with silvery scales that reflect light. The anal fin shows red to reddish-orange coloration, especially in adults, and this feature gives the fish its common name, Red Hook Silver Dollar. The other fins may show dark or reddish tones depending on age, mood, lighting and overall condition. Its mouth is adapted for grazing plant material, and its dentition reflects its membership in the family Serrasalmidae. The overall shape is sturdy, with a rounded dorsal profile and a forked caudal fin suited to continuous swimming. The red anal fin is its most distinctive visual feature. Mature males may have a longer or more colorful anal fin, although these differences are not always reliable in young fish. Its adult size and compact shape make a spacious aquarium important, with no sharp décor or obstructed swimming lanes.

General diet

Myloplus rubripinnis is primarily herbivorous and should receive a diet rich in plant material. Blanched vegetables such as zucchini, spinach, romaine lettuce or shelled peas can supplement algae-based wafers and pellets. Quality commercial foods for herbivorous fish are also suitable, provided that food sources are varied. Small portions of soft fruit or appropriate vegetables may be offered occasionally, but should not replace the main diet. Frozen foods such as daphnia, brine shrimp or small worms can be used as supplements, particularly for young fish or during acclimation, but they should not dominate the diet. Feed small portions because silver dollars eat quickly and can pollute the water. A varied diet supports healthy growth, stronger coloration and natural behavior. For wild-collected individuals, starting with soft vegetable foods may help the transition to prepared aquarium diets.

Essential data

• Native river name: Amazon Basin; the precise waterway varies by population.
• Local name used: Red Hook Silver Dollar; no single local name retained.
• Origin of the scientific name: Genus Myloplus; the species epithet rubripinnis describes the red fin.
• Full-name etymology: Myloplus is a genus name of Greek origin; rubripinnis means red-finned, from ruber and pinna.
• Common/vernacular names: Red Hook Silver Dollar, Redfin Myloplus.
• Family: Serrasalmidae
• Subfamily: Serrasalminae
• Name pronunciation: mye-loh-plus roo-bri-PIN-nis
• Discovery: William Jardine, 1841.
• Maximum adult size: About 25 cm.
• Water temperature and pH: 24 to 28 °C; approximately pH 5.5 to 7.5.
• Waterway appearance and surrounding vegetation: Warm tropical waters, from slow to moderately flowing, bordered by roots, branches, leaf litter and flooded vegetation.
• Typical tankmates: Metynnis hypsauchen, Myleus schomburgkii, Geophagus altifrons, larger peaceful characins, robust loricariids.
• Temperament and behavior: Peaceful, social and active; keep in a group of at least five individuals.
• Water depth: Open-water areas at shallow to medium depth.
• Sexual dimorphism: Adult males may have a longer or more intensely colored anal fin.
• Bottom structure and current: Sandy or muddy bottom with plant debris and roots; low to moderate current.

In your aquarium

To succeed with Myloplus rubripinnis, provide a long, spacious, covered aquarium with properly sized filtration and very well-oxygenated water. Keep them in a group, offer a mostly plant-based diet and avoid tiny or overly aggressive tankmates. Wild-collected fish should be acclimated slowly and quarantined. Stable water conditions, regular water changes and a layout with a large open swimming area are the keys to long-term care.