Silver dollar

Water parameters

Tolerated range for this species. Aim for the middle of each band rather than the extremes.

Tank and habitat

Substrate: any.

Behavior

Plant interaction: destroys most plants.

Feeding

Herbivore that needs a vegetable-heavy diet. Spirulina flake, algae wafers, blanched vegetables (lettuce, spinach, peas, zucchini, cucumber), and aquatic plants (duckweed, hornwort) form the staple. They eat live plants in the tank, which is a feature or a problem depending on your setup. Supplement with occasional frozen bloodworm or brine shrimp for protein, but keep the diet 80%+ vegetable. Feed twice daily. Without adequate vegetable matter, they develop digestive problems and dull coloring. Overfeeding with protein-rich food causes bloating.

Compatibility

• Peaceful despite their size (up to 15 cm diameter). Silver dollars are timid schooling fish that startle easily and can hurt themselves dashing into the glass when spooked.
• Groups of 5+ are necessary. Solitary or paired silver dollars are nervous wrecks that hide, refuse food, and injure themselves on tank decor during panic dashes.
• Good tankmates: large, calm community fish. Oscars, severums, large catfish, and other non-aggressive medium to large species work well. Avoid small aggressive cichlids that nip at their flat body.
• Silver dollars eat plants. Every plant. They're dedicated herbivores that will strip a planted tank bare within days. Never put them in a planted aquarium unless the plants are sacrificial.

Origin and habitat

Silver dollar is a trade name covering several round, coin-shaped characins in the family Serrasalmidae, the group that also holds the piranhas and pacus, though these fish are peaceful herbivores rather than predators. The archetypal one, and the species most often meant, is Metynnis argenteus, endemic to the Tapajos basin in Brazil; it is nearly identical to the wider-ranging M. hypsauchen and the two are constantly confused, while the spotted M. lippincottianus turns up as well. In the wild they live in a range of river, floodplain, and flooded-forest habitats. They are dedicated plant eaters, stripping live aquarium plants within a day or two, so silver-dollar tanks are usually run without soft plants. The body is a flat, silvery disc, sometimes with reddish tints in the fins and throat, and the fish grows to around 14 to 15 cm, with some reports up to 20 cm. They are peaceful, skittish shoalers that do best in groups of five or more; despite the gentle nature, the family's teeth are strong, and M. hypsauchen can deliver a real bite.

Breeding

A benthic egg scatterer. A conditioned group, brought up on a vegetable-rich diet and warmed into the high twenties in soft, dim, acidic water with fine plants, will spawn, often as a school rather than a discrete pair. A female can release a very large clutch, up to around two thousand eggs, which sink to the bottom, so the adults should be removed afterward. The eggs hatch in about three days and the fry are free-swimming roughly a week later, starting on infusoria or liquid fry food before baby brine shrimp. Survival from such a huge spawn is naturally low, and the schooling spawning behaviour makes controlled home breeding hard without a very large tank.

Common problems

The headline issue is that silver dollars eat plants, fast and thoroughly, leaving only the toughest like java fern and some anubias, so a planted display and a school of silver dollars are not compatible. They are also nervous fish that bolt when startled and can hurt themselves hitting the glass or lid, and they jump, so a tight cover, a low-traffic spot, and some floating cover to make them feel secure all help. They need a group of at least five; kept alone or in twos they hide, refuse food, and stress themselves into poor health. Diet-wise they need plenty of vegetable matter, and a protein-heavy diet causes bloating and dull colour.

Bioload

Further reading

• Stocking ideas by tank size