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 common name applied to several species in the genera Metynnis, Myleus, Mylesinus, Myloplus, Mylossoma, and Tometes, all in the family Serrasalmidae. The most commonly sold species are Metynnis argenteus (probably endemic to the Tapajós River basin in Brazil) and M. hypsauchen (found in Amazon and Paraguay river basins and rivers of the Guiana Shield); these two are virtually identical in appearance and care, and are frequently confused in the trade. A third species, M. lippincottianus (spotted silver dollar), also appears occasionally. The family Serrasalmidae includes piranhas and pacu, making silver dollars their close relatives, though Metynnis species are herbivorous or omnivorous rather than predatory. In the wild they inhabit a wide range of freshwater habitats including rivers, streams (both fast- and slow-flowing), floodplains, flooded forests, lakes, and pools. They are enthusiastic plant eaters and will consume live aquarium plants relentlessly; aquarists typically forego live plants in tanks with silver dollars. Metynnis species generally reach 8–15 cm standard length, with a few species reaching up to 18 cm. Round-bodied and laterally compressed, with a uniform silver color that sometimes shows reddish tones in the fins and throat. Peaceful shoalers that should be kept in groups. M. hypsauchen possesses powerful dentition that can cause serious bites.

Breeding

Egg scatterer. Condition a group with a high-quality vegetable diet and frozen food for 2 weeks. The breeding tank needs soft acidic water (pH 6.0-6.5, 4-8 dGH), dim lighting, and fine-leaved plants or spawning mops. Raise the temperature to 28–29°C. A large water change with slightly warmer water can trigger spawning. Males have a longer anal fin with a reddish edge; females are rounder when full of eggs. A single female can scatter up to 2,000 eggs, which sink to the bottom. Remove adults after spawning. Eggs hatch in about 3 days; fry are free-swimming by day 6-9. Start with infusoria or liquid fry food, then graduate to baby brine shrimp. Expect high mortality in such a large spawn. Silver dollars sometimes spawn in schools rather than pairs, which makes controlled breeding in a home aquarium difficult without a very large tank.

Common problems

Plant destruction is the main compatibility issue. Silver dollars and planted tanks don't coexist. The only plants they reliably leave alone are java fern and some Anubias species (too tough and bitter). Everything else is food. They will strip a planted tank bare overnight if hungry. Spook-related injuries are common too; silver dollars are skittish fish that bolt when startled, and they hit tank walls and lids hard enough to injure themselves. A tight-fitting lid is non-negotiable because they jump. Keep the tank in a low-traffic area and avoid sudden light changes. Dim lighting or floating plants on the surface help them feel secure. They need to be in groups of at least five; a lone silver dollar hides constantly and stresses itself sick.

Bioload

Further reading

• Stocking ideas by tank size