Taxonomy & naming
Cyphocharax gillii was described by Carl H. Eigenmann and Clarence Hamilton Kennedy in 1903 from material collected in the Río de la Plata drainage, as part of their broader survey of the fishes of that basin. It sits in the family Curimatidae — small-mouthed, toothless-jawed detritivores and algae grazers, sometimes lumped by aquarists with the headstanders and other "minor characins" — within the order Characiformes.
Within the genus Cyphocharax, C. gillii has been placed in the C. spilurus species complex alongside C. oenas, C. spiluropsis and C. spilurus itself, a group of similarly small, silvery, superficially alike species distinguished mainly by fine details of scale counts, fin-ray counts and pigmentation rather than by any dramatic external feature. That kind of complex is exactly why Catalog of Fishes-level taxonomy matters for this genus: field or photographic identification between the clade members is unreliable, and any name attached to trade specimens should be treated with caution unless it traces to a verified specimen or a specialist source.
Morphology
Cyphocharax gillii is a small, laterally compressed, silvery characin reaching a documented maximum of about 4 in standard length — modest even by the standards of the smaller characins in this family, and nowhere near the size of the pacus, dorado or larger predatory genera covered elsewhere in this atlas. Like other curimatids it has a small, subterminal or underslung mouth adapted for scraping and sifting rather than biting, reflecting its role as a substrate and surface grazer rather than a hunter.
Beyond the FishBase summary measurements and its placement in the C. spilurus complex, detailed published morphological description is sparse; there is not enough in the available sources to responsibly describe fin-ray counts, precise body proportions or colour pattern beyond "small and silvery," and this article does not invent those details.
Habitat
The type locality for Cyphocharax gillii is the Río de la Plata, and its documented distribution covers the Río de la Plata and Río Paraguay drainages of southern South America — a subtropical rather than equatorial part of the continent's river systems. FishBase describes the species as benthopelagic, foraging over stones, roots and plants along the substrate rather than holding in open water.
Specific published water-chemistry data (temperature range, pH, hardness) for this species was not found in the sources consulted here. Given its placement in a subtropical Paraná/Plata drainage rather than the equatorial Amazon, water there is likely to run cooler on average and see more seasonal swing than classic Amazonian blackwater, but that inference is not backed by a species-specific measurement and should not be presented as one; keepers and researchers should treat any specific numbers as provisional until a dedicated source is found.
Feeding
Cyphocharax gillii is an Aufwuchs grazer: FishBase describes it as a bottom-feeder that scoops substrate and searches stones, roots and plants for diatoms, desmids and epiphytic filamentous algae, along with the small organisms living within that biofilm. This is a classic curimatid feeding strategy shared with its relatives the hemiodus and other detritus-grazing characins covered in this group, and it stands in sharp contrast to the predatory diets of Brycon or the dorado elsewhere in the Piranhas & Pacus family.
In practical terms this makes C. gillii an omnivorous grazer rather than either a specialist herbivore or a predator: it takes algae and diatoms directly off surfaces but also ingests the small invertebrates and organic material mixed in with that film, a strategy well suited to continuously sifting through substrate and cover rather than actively chasing food.
Mating
No courtship or pairing behaviour for Cyphocharax gillii was found in the sources consulted for this article. FishBase notes only that the species is "very peaceful" toward conspecifics, which speaks to its general temperament but says nothing about spawning behaviour, sexual dimorphism or courtship displays. Rather than guess from patterns seen in better-studied curimatids, this account leaves the topic as data sparse.
Breeding
Breeding biology for Cyphocharax gillii is not documented in the aquarium or scientific literature consulted here. FishBase makes no mention of reproductive mode, and there is no hobby record of the species being spawned in captivity. Many small curimatids are presumed to be open-water or substrate egg-scatterers with no parental care, by analogy with better-known relatives in Curimatidae and Characidae generally, but that is an inference from the wider family rather than a documented fact about this species, and it should be read as such rather than stated as settled biology.
In the aquarium
Cyphocharax gillii is essentially unknown in the hobby: it does not appear to be traded, and no aquarium care accounts were found among the sources used here. What can be said with confidence, based on its biology, is that a peaceful, small (4 in max), bottom-oriented algae-and-detritus grazer of this kind would in principle suit a well-established, planted or driftwood-furnished community tank with other calm characins, given some open substrate and surfaces to graze — the same broad husbandry logic that applies to its hemiodus and curimatid relatives.
That said, this is inference from its documented biology and its family's general husbandry pattern, not a verified aquarium care record, and prospective keepers should treat any specific tank size, water parameter or diet recommendation for this exact species as unconfirmed until better sources emerge.
Conservation
Cyphocharax gillii has not been formally assessed by the IUCN Red List and is classified here as Not Evaluated. This is unsurprising for a small, obscure, non-commercial curimatid restricted to part of the Río de la Plata and Río Paraguay drainages: it is exactly the kind of species that falls outside both the aquarium trade and the food-fishery pressures that tend to draw conservation assessment attention in this family.
No specific threats or population trend data were found in the sources consulted. As with many single-basin, poorly studied South American characins, the broader pressures on the Río de la Plata system — damming, water-quality change and habitat modification across the Paraná basin generally — are plausible long-term concerns, but there is no species-specific evidence to cite here, and this article does not assert a threat status the data does not support.