Taxonomy & naming
Metynnis fasciatus was described by the German ichthyologist Ernst Ahl in 1931. It sits in the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, and in the genus Metynnis, one of several silver dollar lineages alongside Myloplus, Myleus, Tometes and Ossubtus. Serrasalmid taxonomy has been reworked substantially over the past few decades, with a number of species once lumped into a broader "Myleus" concept redistributed among these genera; the Catalog of Fishes (Eschmeyer, California Academy of Sciences) retains Metynnis fasciatus in Metynnis, and that placement is followed here.
The species epithet fasciatus is Latin for "banded" or "striped", from fascia (a band or stripe), referring to the vertical banding pattern visible on the flanks, particularly in juveniles and in some individuals under stress or during display. The genus name Metynnis is of Greek derivation used by early ichthyologists for this group of deep-bodied characins. In the aquarium trade the common names "striped silver dollar" and "tiger silver dollar" both point to the same banding, and the species is frequently confused with other Metynnis and Myloplus species sold interchangeably as generic "silver dollars".
Morphology
Like other silver dollars, Metynnis fasciatus has a strongly laterally compressed, near-circular body — the source of the common "silver dollar" name — with a small head, a short snout and an unremarkable, non-piranha-like jaw bearing small, close-set teeth suited to grazing rather than shearing flesh. FishBase records a maximum size of 2.5 in standard length, making it one of the more compact serrasalmids in the aquarium trade, well short of the true pacus that can reach a metre.
The body is bright, reflective silver overall, the trait that gives the whole silver dollar group its trade name, with the vertical banding referenced in the species name visible along the flanks — most pronounced in younger fish and fading somewhat with age and in calm, settled individuals. Fins are typically clear to lightly dusky. As in most Metynnis, there is no reliably documented external difference between the sexes outside of breeding condition.
Habitat
Metynnis fasciatus is documented from the Capiuru River basin in the middle Amazon River drainage of Brazil, a South American endemic like every member of its family — never found naturally in Africa or Asia. Within Amazonian drainages, silver dollars of this genus are typically pelagic, mid-water to near-surface schoolers, moving in loose to tight groups through flooded forest margins, backwaters and vegetated river edges rather than holding to open channel water.
Specific water chemistry data for M. fasciatus itself is sparse in the available literature, but genus-level accounts for Metynnis describe warm, soft-to-moderately-hard water in the roughly pH 5–7.5 range and temperatures of about 75–82 °F, consistent with the wider pattern of Amazonian floodplain habitat. Vegetation is a meaningful habitat feature, not incidental — wild fish graze on submerged and marginal plant growth, and the species' distinctive feeding ecology (see "Feeding") is tied directly to living among that plant cover.
Feeding
Metynnis fasciatus is a committed herbivore with omnivorous tendencies, grazing on soft aquatic and marginal vegetation and algae, supplemented opportunistically by small invertebrates, fallen fruit and seed material typical of flooded-forest characins. This is a genuinely important identity point for the species and its relatives: despite belonging to the same family as the piranhas, silver dollars are plant-eaters, not predators, and their small, closely packed teeth are built for shredding leaf tissue rather than cutting flesh.
In the aquarium this translates directly: a diet built around vegetable matter — blanched greens (spinach, cucumber, courgette), spirulina-based flake or pellets, and algae wafers — with only occasional, modest protein such as bloodworm or brine shrimp keeps fish in good condition. Because the species' natural feeding habit is grazing, keepers should expect it to treat any live aquarium plants as a standing food source rather than décor (see "In the aquarium").
Mating
Documented breeding accounts for Metynnis fasciatus describe courtship consistent with the wider silver dollar pattern: conditioned adults in a settled group become more active and closely paired in the lead-up to spawning, typically triggered by softer, more acidic water and a period of rich feeding rather than any elaborate nest-building or courtship display. As in most Metynnis, there is no well-documented external sexual dimorphism beyond a fuller body in ripe, egg-laden females; most breeders identify likely pairs by behaviour and condition rather than appearance.
Breeding
Metynnis fasciatus is an egg-scatterer, not a nest-builder, mouthbrooder or bubble-nest species. Conditioned adults release eggs over fine-leaved plants or spawning mops in a dedicated, well-planted breeding tank with soft, acidic water and subdued lighting; spawning is generally reported to happen in the morning hours. The eggs and resulting fry receive no parental care at all, and adults will readily eat their own eggs, so breeders remove the parents immediately after spawning is observed.
Eggs hatch within a few days under warm conditions, and fry are raised on infusoria before graduating to brine shrimp nauplii and finely crushed foods. The species is occasionally bred by dedicated hobbyists but is not a routine home-tank spawner, and most fish in the trade are still wild-collected or farmed rather than hobbyist-bred.
In the aquarium
Metynnis fasciatus is a peaceful, group-oriented schooling fish that should never be kept alone or in a pair — a real group of five or six or more is needed to keep individuals settled and to spread out any low-level social nipping. Given its modest 2.5 in adult size, it is far more tank-manageable than the true pacus, but genus-level care guidance still calls for a good-sized aquarium (roughly 80 US gal upward for a school) with strong filtration to handle the bioload of an actively grazing, group-living fish, plus open swimming space consistent with its mid-water schooling habit.
The single most important care point is dietary: this is a dedicated plant-shredder, and it will strip soft-leaved aquarium plants in short order. It belongs in a robust, hard-scaped setup with driftwood and tough, unpalatable or plastic plantings rather than a delicate aquascape, unless the keeper is prepared to feed enough vegetable matter to satisfy its grazing instinct and accepts some plant loss regardless. Water in the region of 75–82 °F and pH 5–7.5 suits the genus. It mixes well with larger, robust tankmates such as bigger cichlids and armoured catfish, but is not suited to very small or delicate fish given its size and active feeding.
Conservation
Metynnis fasciatus is assessed by the IUCN Red List as Least Concern (assessed 2018), reflecting its status within a South American endemic genus that is not currently known to face significant range-wide threats. As with many Amazonian floodplain fish, its documented range is narrower and less thoroughly surveyed than that of widespread, heavily fished species, so localized habitat pressures — deforestation, sedimentation and hydrological change linked to damming across the middle Amazon — are relevant long-term considerations even though the species itself is not currently listed as threatened. The aquarium trade in Metynnis species draws on a mix of wild-collected and farmed stock; specific harvest data for this species is not documented in the sources available here.