What type of silver dollars are these

[Byron]

From the single photo is nearly impossible to give a good guess at the species you have. Perhaps the only sure thing to say is that they are not M. maculatus, nor M. lippincotianus. Based on the high body, they are likely M. hypsauchen but better photos would be needed.
Please note that the name M. argenteus (Muller & Troschel 1844), although validly introduced, it is NOT a currently accepted name in Metynnis. Although one of the most commonly used name within Metynnis, specimens of 'argenteus' including the types, belong in several species of Metynnis. Thus it is a messy situation where argenteus is a junior synonym (in part) of several other taxa.

When anyone gets into taxonomy you will always spike my interest. There seems to be some confusion here, as Metynnis argentius is a valid name for a distinct species, not a synonym. E. Ahl described this species in 1923, published in "Eine Revision de Characiniden-Gattung Metynnis," in Ichthyologische Mitteilungen, I. Mitteilungen aus dem Zoologischen Museum in Berlin v. 11 (no. 1): pp. 15-31. Gery (1972) did question if this species might be Metynnis altidorsalis Ahl 1923 and thus the name a synonym, but Taphorn (1992) considered M. argentius as a valid species/name, and everyone since, including Zarsky & Gery (1999) have agreed. The California Academy of Science maintains online Eschmeyer's Catalog of Fishes which has the historical taxonomic data, and this species is deemed valid.