Silver dollar

Quick facts

Adult size

15 cm

Lifespan

can live up to 10 years

Tank zone

mid

Temperament

peaceful

Difficulty

beginner

Schooling

recommended 5+ (critical minimum 3, thrives at 8+)

Water parameters

Temperature

24–28°C

pH

5.5 to 7.5

Hardness

2 to 15 dGH

Tank requirements

Minimum volume

300 L

Minimum length

120 cm

Flow

moderate

Lighting

any

Substrate

any

Open swimming room

needed

Lid

required - jumper

Feeding

Diet: herbivore, feeds primarily at the mid.

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.

Vegetable matter required (algae wafers, blanched zucchini, spinach).

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.

Habitat

The common name 'silver dollar' covers several related species in the genera Metynnis, Mylossoma, and Myloplus, all from South American river systems. The most commonly sold species are Metynnis argenteus and M. hypsauchen. Found in slow-moving rivers, tributaries, and flooded forest areas in the Amazon and Orinoco basins, where they feed on aquatic vegetation, fruits, and seeds. The body is laterally compressed and almost perfectly round (the 'silver dollar' shape) with a bright silver coloration that reflects light. Some species develop red patches on the anal fin or body. Adults reach 12–15 cm in diameter, which, combined with their need for groups, means they require large tanks (300 L for a school). The species complex has been in the hobby since the early 20th century. Commercially bred stock is standard, though wild-caught specimens of rarer species appear periodically. They're related to piranhas (family Serrasalmidae) but are herbivorous and completely peaceful.

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 6.0-7.0), dim lighting, and fine-leaved plants or spawning mops. Males may develop small hooks on the anal fin and show slightly darker coloring. Spawning happens in groups: several males and females scatter 500-2000 eggs among plants. Eggs are semi-adhesive and photosensitive; keep the tank dark. Remove adults after spawning. Eggs hatch in 3-4 days. Fry become free-swimming in another 3-4 days and eat fine vegetable matter (spirulina powder, green water, crushed algae wafer) and baby brine shrimp. Growth is moderate. Breeding is achievable in large tanks but the space requirements make it impractical for most hobbyists.

Common problems

Plant destruction is the main compatibility issue. Silver dollars and planted tanks don't coexist. The only plants they don't eat are java fern (too tough) and some Anubias species (also tough-leaved). Everything else is food. Spook-related injuries are the main health concern. A startled school of silver dollars can dash into the glass hard enough to damage their blunt noses or crack a heater guard. Minimize sudden movements and loud noises near the tank. Cover the back and sides with dark background material to reduce reflections. Ich appears in new or stressed fish; standard treatment works. Internal parasites (tapeworms) occasionally appear in farm-bred stock, causing weight loss despite eating. Treat with praziquantel.

Bioload

Bioload coefficient: 5.0 (large herbivorous characin; eats constantly and produces waste to match).

Bioload coefficients are calibrated against the neon tetra as the anchor (1.0). See the methodology page for the formula and how each value was derived.

Further reading

• How to cycle a fish tank
• Stocking ideas by tank size