Mono argentus (Silver Moony Fish)

Mono argentus

Last updated on August 24th, 2022 at 01:53 pm

The silver moony fish, also known as the Mono Argentus, mono angelfish, silver moonyfish, silver moony, Monodactylus argenteus, butter bream, or diamondfish, is found in freshwater or brackish water environments across Japan, especially in streams and shallow ponds.

The fish has both silver scales and silvery grey skin, which gives it its name; moony fish are also sometimes called moonfish, moorfish, or moonies. While popular in Japan, the fish is considered an invasive species in some parts of North America, though it has not been listed by the World Conservation Union or Fish & Wildlife Service as being harmful to local ecosystems.

They are found around the Mediterranean and as far north as the Baltic Sea, with evidence suggesting that they might also be found in the North Atlantic around Iceland. They primarily inhabit the salty waters of coastal lagoons, bays, and estuaries, where they are sometimes spotted in schools swimming near the surface of the water and even leaping out of the water to capture an insect or small fish that has fallen in.

Origin and descriptions

Mono argentus

This fish is found in parts of South America, particularly in Brazil and Venezuela. It can be distinguished by its silver skin, sharp teeth, and spikes on its body (not unlike a porcupine). Its name means silver moon in Latin. Also called a moonfish, it got its name from both of these traits because it only comes out to hunt at night.

The average Mono Argentus is between 6 and 11 inches long but can grow as large as 12 inches long. They are extremely territorial and will attack any other animal that enters their territory. They are also very aggressive towards humans if they come too close to their homes or nest.

There have been several reported attacks on humans who were swimming or wading in the water where they live, so extreme caution should be taken when around them.

Indian Glass Fish (Parambassis ranga)

Species profile

Mono argentus belongs to the family Monodactylidae. They are endangered species that can be found in deep trenches off of Australia. These fish only grow to be about 11 inches long and have a silver tint as well. These fish are extremely rare, so when it comes to fish collectors, every single one counts.

Any individual in possession of a Mono Argentus is required by law to report its location to protect against extinction. It is also unlawful for these fish to be removed from their natural habitat without consent from both state and federal authorities. This makes them very difficult to obtain for private aquariums or any other purpose outside of scientific research.

Common names

Mono argentus is also referred to as silver moony fish, Monodactylus argenteus, silver moonyfish, silver moony, mono angelfish, diamondfish, or butter bream.


The Silver Moony is native to Southern Argentina, particularly in Puerto San Julian where it is abundant. This fish typically lives in shallow water near coastal regions, but has been found as deep as 30 meters. These creatures are primarily nocturnal (active at night) and prey on small crustaceans. Their large eyes give them an exceptional vision that enables them to pick out small animals from a distance.

They have evolved to live in low-light environments, which explains their unique silver coloration. They also have rows of bioluminescent cells along their sides that glow when disturbed or agitated, helping them blend into their environment even more effectively.

Mono argentus size

Mono argentus

These species of fish can grow up to 11 inches (27 cm) in length in the wild, but they may generally not get bigger than 6 inches (15 cm) in length when kept in captivity.

Tank size

The minimum recommended tank size for mono argentus is 35 gallons (132 liters)

Tank requirements

The Silver Moon fish, also known as Mono Argentus, is a medium-sized tank fish. It needs at least a 35 gallons tank with plenty of plants and rocks to hide in. If you want to keep it with other fish make sure they don’t attack it.

African Monodactylus Sebae (African moony)

The aquarium should be kept in a location that stays between 70 and 80 degrees Farenheit all year round. Don’t put it in an area where there are large temperature fluctuations or near a window or drafty door. Also, make sure to clean your tank regularly so its water doesn’t get cloudy from algae buildup.

Water hardness should be inversely proportional to water temperature. In other words, as the water temperature rises, water hardness falls and vice versa. This is due to a number of factors including; dissolved oxygen content (as the temperature rises oxygen becomes less soluble) and nitrogenous waste products (the more active a fish is, the more waste it produces).

The lower end of these parameters is for adult specimens while younger specimens will require slightly higher values to prevent stunting or growth deformities caused by fluctuating levels.

Mono argentus tank mates

Mono argentus can be housed with other peaceful fish that are roughly its size. Smaller fish will have a hard time competing for food and hiding places, while larger fish may bully it. Good tank mates include species of Gobies (only brackish species), Scats, larger Mollies, and Archerfish.


Mono argentus

Although these species of fish can spawn in the ocean, differentiating both sexes from each other and their breeding pattern is unknown.

Are they aggressive or peaceful?

While Mono Argentus can be territorial, they are not aggressive or dangerous. They prefer to keep their distance from other fish, so they aren’t considered a threat. In fact, despite their predatory nature, when kept in captivity, Mono Argentus seem more inclined to scavenge than hunt for food.

Mono argentus care

Mono argentus

Silver moony fish require an aquarium of at least 35 gallons with a sand or fine gravel substrate and plants. They are relatively easy to care for as long as they have enough space to swim around. A good quality filter and regular water changes will keep your moony fish happy and healthy.

Green Spotted Puffer Fish - Tetraodon nigroviridis

I recommend feeding a high-quality pellet food, but supplement with vegetables, brown algae pellets, spirulina tablets, and live foods like bloodworms when possible. It is very important that you do not overfeed these fish. If you notice your moony eating a lot more than usual, it may be trying to tell you something – take it out and check its belly!

Also, note that silver moony fish are nocturnal so their activity level during daylight hours is limited; if kept in a brightly lit tank, they may become stressed.

What they eat

Silver moony fish eat small insects and crustaceans. They are omnivores, so they will also eat algae, plankton, small fish, mussels, and barnacles. They have to be kept in a tank with food sources or they will go hungry and even starve to death. This can lead to illness as well. To avoid these issues try feeding your silver moony a diet of different types of meaty foods like shrimp and chopped seafood as well as live brine shrimp or mysis shrimp.


Mono Argentus can live up to 7-10 years when properly taken care of.

Parasites and diseases

The silver moony is prone to several different parasites and diseases. Though it seems to be easy enough to care for, it is still susceptible to illnesses like flukes, ich, and even pneumonia. The best way to avoid illness in your fish is by providing a healthy diet and a clean environment.

You should also keep watch for Slimy patches of skin symptoms, dull or clamped fins; open sores on their body or eyes. These are all signs that something is wrong.

It’s important to treat any disease immediately so as not to endanger your entire tank. This will help ensure that you don’t lose any other fish in addition to mono argentus.


Predators of fish include birds, mammals, reptiles, and even other fish species. As with most animals in nature, most predators are looking for an easy meal and thus will often take advantage of prey that aren’t fully aware of their surroundings.

Banded Archerfish (Toxotes jaculatrix)

Some examples of mono argentus predators are

  • Birds: seagulls, cormorants, pelicans, and herons.
  • Mammals: otters, dolphins, and whales.
  • Reptiles: sea turtles and alligators.
  • Other fish species: sharks, barracudas, and tuna.

Do they make good pets?

Mono argentus makes for a good pet if you’re looking for a fish that is not too hard to take care of. They don’t do well in colder water and prefer warmer waters between 65-75 degrees Fahrenheit. Mono argentus are primarily nocturnal, so they’ll often sleep during the day and come out at night. They usually like to eat crustaceans, insects, algae, and plants.