confirmed as a species from Argentina:
Brycon hilarii (Valenciennes, 1850)

published in:
Lima, F.C.T. (2017):
A revision of the cis-andean species of the genus Brycon Müller & Troschel (Characiformes: Characidae).
Zootaxa 4222 (1): 1-189

abstract (from publication):
A revision of the cis-andean species of Brycon, with the exception of the Brycon pesu species-complex, is presented. Twenty-one Brycon species (including B. pesu) are recognized from cis-andean river systems: … Brycon orbignyanus (Valenciennes), from the rio Paraná and rio Uruguai basins, Brazil, Paraguay, Argentina, and Uruguay; Brycon hilarii (Va¬lenciennes), from the rio Paraguai, middle rio Paraná, and upper rio Amazonas basins, Brazil, Paraguay, Argentina, Peru, and Ecuador … All species are redescribed and illustrated, and a key to the species is provided. Comments on the diagnosis of the genus Brycon, the biogeography of the cis-andean species, and their current conservation status, are presented.

distribution (from publication):
Rio Paraguai and middle rio Paraná basins in Brazil and Paraguay, and upper Rio Amazonas basin in Peru, Ecuador, and Colombia. Brycon hilarii is recorded for the rio Paraná below the now submerged Sete Quedas falls, the previous limit between the upper and lower portions of the rio Paraná. Photographs of specimens caught in the rio Paraná between Paso de la Patria and Itatí, Corrientes, provided by M. Ribeiro and F. Baena, and juveniles in the area of Rosario (province of Santa Fé, 32°57’S, 60°40’W), provided by F. Baena, testify the occurrence of Brycon hilarii also in Argentina, from where the species was in fact previously recorded by Menni (2004: 74, as B. microlepis) but not in other check-lists of the ichthyofauna of Argentina (López et al., 2003; Koerber, 2009). …

It is striking that since the note of Baena in 2011 (below for download) it seems to not have been possible to provide preserved specimens from Argentina to the current author and include the species doubtlessly in this paper.
Consequently, Lima does not include Argentina as area of distribution in the first phrase of the above text and the respective map. As some collection localities are found in the area of Asunción a distribution in Argentina is considered as confirmed. To deposit voucher specimens in an ichthyological collection is still necessary.