Metynnis argenteus is a species of serrasalmid fish endemic to the Tapajós River Basin in Brazil. It is one of the species known in the aquarium trade as the "silver dollar". M. argenteus is generally considered the archetypal silver dollar, although it and the very similar M. hypsauchen frequently have been confused.
Round-bodied and laterally compressed, under normal conditions M. argenteus is a uniform silver colour, sometimes showing vague red colouration in the fins and around the throat. Commonly seen for sale as juveniles at around two inches long, they are peaceful shoalers. Fully grown females in captivity can reach a length of between five and six inches while males tend to be marginally smaller. In good condition, females tend to be fuller in the belly than males, while males develop marginally longer fins as they grow older. During courtship and breeding, males develop two large, very prominent black spots, one above the other, just behind the base of the pectoral fins, the red colouration of the fins deepens and contrasting black borders appear. Some male specimens may also display other dark marbling on the flanks. Females display little, if any, variation in colour during courtship and breeding.
Metynnis argenteus, like all other known Metynnis, are mainly herbivores, but also take animal matter. They will readily and eagerly accept dried flake food as well as live and frozen foods like bloodworm, shelled shrimps, rinsed tuna, daphnia and brine shrimp. Their diet should be varied to keep them in good condition. They will nibble at sliced cucumber and show considerable interest in shelled peas and carrots, boiled potato and small chunks of fresh and tinned fruit. They will hack at live plants all day long to the extent that aquarists routinely forego live plants in a tank containing silver dollars and resort to decorating with plastic plants instead.
