not a species from Argentina:
Gymnogeophagus gymnogenys (Hensel, 1870)

published in:
Malabarba, L.R., M.C. Malabarba & R.E. Reis (2015):
Descriptions of five new species of the Neotropical cichlid genus Gymnogeophagus Miranda Ribeiro, 1918 (Teleostei: Cichliformes) from the rio Uruguay drainage.
Neotropical Ichthyology 13 (4): 637-662

abstract (from publication):
Gymnogeophagus is a Neotropical cichlid genus distributed in the río Paraguay, rio Paraná and rio Uruguay drainages and also in the coastal rivers of Uruguay and southern Brazil. Its monophyly is supported by two derived features: the absence of supraneurals and the presence of a forward spine in the first dorsal-fin pterygiophore. Herein, five new species of Gymnogeophagus are described from middle to upper tributaries of the rio Uruguay drainage and from the rio Negro. All these new species belong to a clade, which includes G. gymnogenys, easily recognized by sharing two synapomorphies: the absence of an oblique bar between the dorsal border of the eye and the nape, and the possession of a black bar originating in the dorsal contour near the dorsal-fin origin and directed downward and backward on the dorsum. A key to all species of Gymnogeophagus is provided.

distribution (from publication):
… we herein describe five new species distributed from the rio Quaraí, a tributary of the middle rio Uruguay, to tributaries of the upper rio Uruguay, and restrict the distribution of G. gymnogenys to the laguna dos Patos and rio Tramandaí drainages in Rio Grande do Sul, Brazil and Uruguay.