Taxonomy & naming
Myloplus arnoldi was described by the German ichthyologist Ernst Ahl in 1936 as Myleus arnoldi. Serrasalmidae — the family uniting piranhas, pacus and silver dollars within the order Characiformes — has since undergone a substantial taxonomic overhaul, and the old genus Myleus was split apart; a large block of its species, including this one, was reassigned to Myloplus. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) now treats Myloplus arnoldi as the valid combination, and older literature, shop listings and some online references that still use "Myleus arnoldi" are referring to the same fish under its superseded name.
In the aquarium trade the species circulates almost entirely under vernacular names rather than its scientific one — Arnold's pacu and, more commonly, the yellow fin red hook silver dollar, the latter describing both its fin colouring and the hooked shape of the mature male's anal fin. It is one of many silver-dollar-type serrasalmids that are easily confused with one another in the trade; positive identification rests on the combination of fin colour pattern and body proportions rather than casual appearance.
Morphology
Like other silver dollars, Myloplus arnoldi has a strongly laterally compressed, disc-shaped body with a small head and a mouth adapted for grazing rather than predation. FishBase records a maximum published length of 13 in total length and a weight of 1.3 kg, making this one of the larger silver dollars kept in the hobby — considerably bigger than the diminutive body most specimens show at the size they are typically sold.
The species' calling card is its finnage: the dorsal, anal and caudal fins carry a bright yellow ground colour edged and washed with red, giving the fish its "yellow fin red hook" trade name. In mature males the rear of the anal fin elongates into a distinct hook-shaped lobe, a feature used to sex individuals in a group (see Mating). The body itself is silvery and mirror-bright, typical of the silver-dollar form, with the coloured fins providing the main visual interest against an otherwise plain flank.
Habitat
Myloplus arnoldi is endemic to Brazil, recorded from the Xingu, Tocantins, Trombetas and Jari rivers and the Amazon basin more broadly — all clearwater-to-blackwater tributary systems draining the Brazilian Shield into the lower Amazon. Within these rivers it is associated with the vegetated margins, flooded forest and slower stretches typical of silver dollars generally, where it grazes on submerged and marginal plant matter, algae and fruit falling from the canopy.
Specific published water-chemistry data for this species were not found in the sources consulted for this article; keepers should default to the warm, soft-to-neutral conditions typical of Amazonian tributary silver dollars rather than assume extreme blackwater parameters without direct evidence.
Feeding
Myloplus arnoldi is primarily herbivorous, grazing on aquatic vegetation, algae and fallen fruit and seed matter in the wild, consistent with the broader dietary pattern of silver dollars in Serrasalmidae. In the aquarium it should be offered a vegetable-forward diet — blanched vegetables such as courgette, cucumber and spinach, spirulina-based flake or pellet, and algae wafers — supplemented with a modest amount of protein such as quality omnivore pellets.
The species' grazing habit is not incidental to its care: a hungry group will readily turn to any live plants in the tank as a food source, and feeding a plant-appropriate diet in sufficient quantity is the main way keepers avoid a planted layout being stripped (see "In the aquarium").
Mating
The clearest external sexual difference in Myloplus arnoldi is the male's anal fin, which develops the elongated, hook-shaped extension referenced in the species' hobby name as it matures; females retain a shorter, more evenly rounded anal fin. Beyond this fin shape, detailed accounts of courtship behaviour for this species were not found in the sources consulted, and this article does not speculate beyond what the cache supports. Conditioning a group on a rich, varied diet and maintaining stable, warm water are the general prerequisites hobbyists report for encouraging spawning behaviour in silver dollars of this type.
Breeding
Silver dollars in this group are egg-scatterers, and Myloplus arnoldi follows that pattern: a conditioned pair or group releases eggs over fine-leaved plants or plant-like structure in soft, acidic water, with no nest built and no parental care afterward. The eggs and resulting fry are not guarded, and in a community or mixed-species setting adults and other tankmates will readily eat scattered eggs, which is the main reason home breeding is only occasional rather than routine.
Hobbyists who have bred related silver dollars generally report success by conditioning a group heavily on vegetable and live/frozen foods, softening and acidifying the water to encourage spawning, and providing a fine-leaved spawning mop or plant thicket, then removing the eggs promptly to a separate rearing container away from adult predation. Specific published breeding accounts for Myloplus arnoldi itself were not found in the sources consulted; data sparse.
In the aquarium
The single most important planning fact for this species is its adult size: FishBase gives a maximum of 13 in, well beyond the roughly 7 in juvenile size most specimens are sold at and far larger than many buyers expect. A mature specimen or, better, a proper school needs a large aquarium — aquarium-trade sources recommend a minimum of around 100 US gallons (roughly 100 US gal) for a group — with plenty of open swimming space, since this is a schooling fish that does best kept in numbers of six or more rather than alone or in pairs.
Myloplus arnoldi is peaceful and sociable toward tankmates of a similar size and temperament, but it is a determined plant-shredder: soft-leaved species will be grazed down quickly, so keepers wanting greenery in the tank should stick to tougher, less palatable plants such as Java fern and Anubias, or accept an unplanted, hard-scaped layout with driftwood and rockwork instead. This is a robust-tank fish, not an aquascaper's centrepiece.
Conservation
Myloplus arnoldi is assessed by the IUCN Red List as Least Concern, and no specific threat data emerged in the sources consulted for this article, consistent with a species distributed across several major Brazilian river systems (Xingu, Tocantins, Trombetas, Jari) rather than confined to a single vulnerable watercourse. As with other Amazonian tributary fish, the longer-term watch items are the general pressures facing these rivers — damming (notably on the Xingu and Tocantins, both subject to major hydroelectric development), deforestation-linked sedimentation and water-quality change — even though the species itself is not currently considered threatened.