Taxonomy & naming
Myloplus rubripinnis was described by Johannes Müller and Franz Hermann Troschel in 1844 and belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes. For decades it was carried in the literature under the older, broader genus Myleus, and much hobby writing and older care sheets still use that combination; a taxonomic revision by Jégu and colleagues in 2004 split that catch-all genus, moving this species and its relatives into Myloplus. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) recognises Myloplus rubripinnis as the current valid combination, and that is the name used here.
The species carries a two-part scientific name in which the epithet rubripinnis is Latin for "red-finned," a direct reference to the vivid red anal fin edged in black that is the fish's most obvious field mark and the source of its common trade name, red hook silver dollar. In the hobby it is sometimes confused with other red-finned Myloplus and Myleus species, and reliable identification rests on the precise shape and extent of the anal-fin colouration alongside body proportions.
Morphology
Like other silver dollars, Myloplus rubripinnis has a strongly laterally compressed, near-circular body that flashes silver in open water — a shape built for schooling and for slipping quickly through cover rather than for pursuit predation. FishBase records a maximum length of 16.5 in total length, with SeriouslyFish citing roughly 10 in standard length as a more typical adult size; either figure describes a substantially large, dinner-plate-sized fish, not the compact "silver dollar" size implied by some retail labelling.
The standout feature is the anal fin: a hook-shaped, extended fin flushed deep red and trimmed in black, contrasting sharply against the fish's bright silver flanks. FishBase notes the species carries two rows of premaxillary teeth with molariform (grinding) cusps, dentition built for crushing seeds, fruit and tough plant material rather than shearing flesh — a useful anatomical tell that distinguishes silver dollars from their piranha relatives in the same family.
Habitat
Myloplus rubripinnis is broadly distributed across the Amazon basin and the Orinoco, with records from Brazil, Peru, Ecuador, Colombia, Venezuela, Guyana, Suriname and French Guiana. Within that range it favours slower-moving stretches of river with dense marginal and submerged vegetation, the kind of structurally complex, food-rich habitat that supports its grazing lifestyle and gives a schooling fish cover from predators.
Water conditions across its range run warm and moderately soft to neutral: SeriouslyFish gives roughly 73–81 °F and pH 6.0–7.0, consistent with typical Amazonian and Orinoco lowland river chemistry rather than extreme blackwater. Seasonal flooding likely expands its access to flooded-forest vegetation for at least part of the year, in keeping with the plant-heavy diet documented for the species.
Feeding
Myloplus rubripinnis is a herbivore, grazing the leaves of submerged and marginal aquatic plants along with algae, and FishBase's dental description — molariform teeth suited to crushing seeds — points to fallen fruit and seeds as a further wild food source, consistent with other Serrasalmidae that exploit the flooded forest. It is an avid, persistent plant consumer rather than an occasional nibbler.
In the aquarium it takes a similarly plant-forward diet: algae wafers, spirulina-based foods, blanched or fresh vegetables such as courgette, cucumber and peas, alongside some protein in the form of bloodworm and brine shrimp. Keepers should expect the fish to treat any live aquarium plant as a menu item, and feeding a vegetable-rich diet consistently is both a welfare need and the only realistic way to reduce (not eliminate) pressure on tank furnishings.
Mating
Documented information on courtship behaviour in Myloplus rubripinnis is sparse. A hobbyist-reported breeding account (SeriouslyFish's care-guide sourcing) describes a pair separating out and becoming noticeably paired within a conditioned school, a pattern broadly consistent with other schooling silver dollars, but no detailed courtship display or trigger conditions have been documented for this species specifically. Sexual dimorphism is only suggested, not scientifically confirmed: some sources point to differences in anal-fin shape between presumed males and females, but this has not been verified in the literature, and most keepers cannot reliably sex the species by eye.
Breeding
Myloplus rubripinnis is, like other silver dollars, an egg-scatterer: eggs are released over fine-leaved plants with no nest built and no parental care afterward. It is rarely bred in the aquarium. SeriouslyFish states outright that captive breeding "has not been achieved" in any consistent, repeatable way, and a separate hobby source records a single documented successful spawning — by a hobbyist in Dayton, Ohio, who reported roughly 50 fry from a 150-gallon tank — as a notable exception rather than the norm.
Where spawning has occurred, the pattern fits the wider silver-dollar mode: soft, acidic water and dense fine-leaved vegetation appear to be the conditions associated with any success, with eggs scattered rather than deposited at a guarded site and no evidence of the parents defending eggs or fry. Prospective breeders should treat this species as a genuine challenge rather than a routine hobby project.
In the aquarium
Myloplus rubripinnis needs to be sized for its real adult length — up to roughly 10–16.5 in depending on the measurement standard — which rules out anything but a large aquarium; SeriouslyFish recommends a densely furnished tank of 75 US gallons (roughly 75 US gal) or more even before accounting for a full-grown group. It is a schooling fish and should be kept in a group of six or more: a solitary or thinly-kept specimen is a stressed fish, and a proper school also spreads out any squabbling over territory and food.
The single most important care note is that this species will eat live aquarium plants with real enthusiasm — it is not a fish for an aquascape. Keepers who want greenery in the tank should rely on tough, fast-growing or artificial plants and expect ongoing losses; driftwood, roots and open swimming space suit the fish better than a planted layout. Water should be kept warm (73–81 °F) and moderately soft to neutral (pH 6.0–7.0), with strong filtration for a large-bodied, actively grazing group. Diet must be vegetable-forward — algae wafers, spirulina, blanched vegetables — supplemented with modest protein, both for the fish's health and to somewhat reduce (never eliminate) its appetite for the plants around it.
Conservation
The IUCN Red List assesses Myloplus rubripinnis as Least Concern, an assessment carried out by G. L. Brejão and published in 2023 (assessment ID 49829569, DOI 10.2305/IUCN.UK.2023-1.RLTS.T49829569A85567244.en). This reflects the species' wide distribution across the Amazon and Orinoco basins and the absence of evidence for range-wide population decline.
As with many widespread Amazonian and Orinocan Serrasalmidae, the main long-term watch items are basin-scale pressures — deforestation-linked habitat and water-quality change, and damming affecting flow and flooding regimes in parts of its range — rather than any documented species-specific threat. The aquarium trade in this species draws on wild-collected stock given the near-absence of established captive breeding, which is worth keeping in mind even though the species itself is not currently threatened.