Silver mono

Monodactylus argenteus

Also known as Silver moony, Mono argentus, Fingerfish, Malayan angel

BrackishSaltwaterFish

Silver mono (Monodactylus argenteus) — Lawrencekhoo · CC BY-SA 4.0

Care at a glance

Temperature: 75–82°F (24–28°C)
pH: 7.5–8.5
Adult size: 9.8 in (25 cm)
Minimum tank: 106 gal (400 L)
Temperament: Mostly peaceful
Social behavior: Schooling
Swim level: Middle
Difficulty: Difficult
Breeding: Hard
Rarity: Common
Reef safe: Not reef safe
Origin: Indo-Pacific coastal waters, estuaries, and mangroves from East Africa to the western Pacific
Family: Monodactylidae

Overview

Keep them in a shoal of at least five or six, since lone or paired monos turn skittish and nippy and a proper group settles them down. That group of large, active fish needs a big tank, on the order of 400 liters or more with a long footprint. They swim in the middle and upper water column and are mostly peaceful, but adults get pushy and will nip slower or smaller tankmates, so pair them only with robust brackish company like scats, archers, and larger brackish gobies. Hold the water alkaline and hard at a pH of 7.5 to 8.5 and 24 to 28 C, and raise salinity gradually as they grow rather than in sudden jumps. Feed them as omnivores with a mix of meaty frozen foods and plenty of greens, since a veg-poor diet causes problems over time.

In the wild

Silver monos live in coastal estuaries, harbours, and mangrove creeks across the Indo-Pacific, from East Africa to the western Pacific, where salinity rises and falls with the tide. This is the key to keeping them: the salt requirement climbs as the fish ages. Small juveniles do fine in low-end brackish around 1.005 to 1.010 specific gravity, but adults want high brackish to near-marine water, roughly 1.018 and up, so plan for a system that can hold marine-grade salinity down the line. Give them a large, open, well-filtered tank with alkaline, mineral-rich water at a pH of 7.5 to 8.5 and a temperature of 24 to 28 C. These are big, fast, deep-bodied swimmers, so length and volume matter more than decor.

Good to know

This is a deep, disc-shaped fish in the family Monodactylidae, reaching about 25 cm in captivity. It is euryhaline, meaning it handles a wide salinity range, but it does not stay in freshwater long-term the way the trade sometimes implies. Spawning is pelagic in the wild and effectively does not happen in home aquaria, so nearly all fish are wild-caught estuarine juveniles.

Did you know

They are one of the classic estuary fish that walks the line between fresh, brackish, and marine over a single lifetime. Shops often sell tiny juveniles sitting in nearly fresh water, which makes them look like an easy community fish, but keeping one to adulthood means slowly turning a freshwater tank into what is essentially a marine one.