Silver Moony (Monodactylus argenteus)

The silver moony, often sold as the mono or diamondfish, is a tall, disc-shaped fish that flashes like a mirror under the lights. A school of moonies moving together is one of the most striking sights in a large aquarium.

It is often sold in freshwater sections, but Monodactylus argenteus is a brackish and marine fish. Juveniles cope with fresh water for a while; adults need salt water to stay healthy. Adults reach about 20 cm.

Where silver moonies come from

The species ranges across the Indo-Pacific, from East Africa to Samoa. Young fish live in mangroves, estuaries and the lower reaches of rivers, sometimes in almost fresh water. As they grow, they move out to coastal reefs and harbours.

Appearance and behaviour

The body is silver with yellow on the tips of the dorsal fin and tail. Two dark bars cross the head, one through the eye. Older fish lose some of the yellow.

Moonies are schooling fish. Kept singly or in pairs, they become nervous and often bully each other. Keep a group of five or more so aggression is spread out.

Tank setup

A group needs at least 400 litres and a long tank with plenty of open swimming space. Use sand, a few rocks or pieces of driftwood, and strong filtration. They are fast swimmers and can jump, so a lid is essential.

Water parameters

• Temperature: 24–28°C
• pH: 7.5–8.5
• Salinity: specific gravity 1.005–1.015 for juveniles, rising to full marine (1.020–1.025) for adults

Raise salinity slowly, no more than 0.002 a week. Measure it with a refractometer, not a swing-arm hydrometer.

Feeding

Moonies are greedy omnivores. Offer quality flakes and pellets, frozen mysis, brine shrimp and krill, with some vegetable-based food. Feed two or three small meals a day rather than one large one.

Tank mates

Good brackish companions include the spotted scat, archerfish and larger gobies. In a marine tank, they suit robust fish such as damselfish and larger wrasses. They will eat small fish and shrimp.

Breeding

Silver moonies have not been bred reliably in home aquariums. In the wild they spawn in the sea, and the young move into estuaries. Aquarium fish are wild-caught or farmed.

Common problems and lifespan

Fish kept in fresh water long term become prone to fungus, fin rot and shortened lives. Stress from small groups is another common problem. In brackish or marine water with a good group, they live ten years or more.