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Related FAQs: Monos

Related Articles: Scats and monos; Old favourites and new species for the brackish water aquarium by Neale Monks, Other Brackish Water Fish Groups

/The Conscientious Marine Aquarist

Marvelous Monos; the Moonfishes, Finger Fishes, family Monodactylidae

Bob Fenner

  Monodactylus argenteus

With names like Moonfish and Fingerfish you know the Monos have got to be some spaced-out aquarium fishes. And indeed, behaviorally and structurally they are.

Though Monos are make periodic incursions into the realm of brackish (fresh & salt mixed waters) in the wild, these are truly marine fishes that generally do well only when maintained in full-strength seawater or carefully maintained hard, alkaline brackish. The "secret" to their optimum health in captivity are besides the aforementioned suitable, stable environment, is proper/sufficient diet.

Classification: Taxonomy, Relation With Other Groups

The family of Monos comprises two genera, Monodactylus and Schuttea of four species. The genus Monodactylus sports three of the species, two being regularly offered in the trade. Of the others, examples of Monodactylus falciformes and Schuttea can be found in Axelrod, Burgess and Hunziker's Atlas, Volume 1, Marine Fishes.

Monodactylus argenteus (Linnaeus 1758), the common Mono, aka Malayan Angel fish, Silver Moony,  is broadly equilateral triangle-shaped; a beautiful shimmering silver fish with yellow highlighted areas immediately in front of the dorsal and anal fins. Indo-Pacific in distribution, including the Red Sea. To more than eight inches in height. These ones in the Long Beach Aquarium of the Pacific.

Monodactylus sebae, the African mono, or finger fish, is much more deep-bodied, and bears four stark black vertical bars on its body. It gets progressively taller with age, growth., the African mono, or finger fish, is much more deep-bodied, and bears four stark black vertical bars on its body. It gets progressively taller with age, growth. (Cuvier 1829), the African Mono, or Finger Fish, is much more deep-bodied, and bears four stark black vertical bars on its body. It gets progressively taller with age, growth. Eastern Atlantic, Senegal to Angola coastal and Canary Islands distribution. To about ten inches in height. Shown are tank bred and reared juvenile of one inch height and an individual of five inches.

Natural and Introduced Range

The family Monodactylidae are found in western Africa and the Indo-Pacific. The two principal aquarium species, Monodactylus sebae and Monodactylus argenteus are wild-collected as well as tank-bred and raised at fish farms in the Far East and Florida (and sporadically elsewhere).

Shape, Size: 

Monos have strongly laterally (side to side) compressed bodies of a distinctive shape; swimming in a swagging motion with their long-based, rear-oriented dorsal and anal fins. Adding to their streamlined appearance, Monodactylus lack pelvic fins entirely. 

The two commonly encountered species attain eight inches in height, though specimens of more than half that are rare in captivity.

Selection: General to Specific

The best small (1-2" or less) Monos are farm-raised; unsurprisingly, these adapt more readily to captive conditions. Often, tank-bred and raised specimens are half the cost, and more than triple the survival rate of wild caught. How to tell which is which? Ask your supplier, and take a look at their stock. Non-wild Monos are typically uniform in size, color, behavior, compared with wild stock. A further benefit for non-marine aquarists, tank-raised specimens tend to be brackish-fresh acclimated from the get-go.

Larger Monos (and many small ones) are collected in seawater, or polluted fresh. Some do well in acclimating to big tanks (see below), many do not. Take care to thoroughly discuss source and replacement matters with whoever is providing you. My advice is to wait a good week or two after arrival before making a purchase, and then to obtain a small, odd-numbered school (these are social animals), and "batch-process" them, dips/quarantine/introduction to the main system, all together.

Color and cut marks are telling. Select for specimens with good, complete silvery scaling; and clear, shiny eyes. Specimens darken for several reasons, fear, unacceptable water conditions, cover of night... during daytime they should be out and shiny bright.

Should the dealers specimens be less-than exemplary, don't be shy about requesting that more be "special ordered", or pass them by for later consideration.

A final word, regarding netting. How fast are these fishes? FAST. Don't thrash the system and its occupants trying to catch them; remove ALL THE DECOR, and carefully and skillfully wield two nets, one for directing the other for scooping upwards once the fish are cornered. Trust me on this.

Environmental: Conditions


If not going with a totally marine set-up, watch out for the vagaries of partially concentrated seawater; principally be wary of salinity "drift" from evaporation and salt depositing. Maybe it goes without writing that your gear must be "rust-proof" as well if you are adding salt.

For physical, chemical and biological stability's sake, the system should be as large as practical, at least forty gallons says I. Smaller tanks are inherently too easily given to crashes, and don't provide cruising and growing space.


A high, and stable specific gravity should be attempted; the osmotic shock and concomitant stress/bioadaptation to rapid changes in salinity are to be avoided. Monos can/will adapt to less salty water but such changes should be gradual, a few thousandths a week.

Synthetic or natural seawater of a constant 1.021-1.025 specific gravity is ideal for Monos. They can, and some populations do, live in more freshwater (to feed and breed), but these fishes are at their best color, behavior and longevity in full-concentration seawater.

If you're not going the totally saltwater route: A timely mention of the need to make sure other tank specimens can tolerate salt levels. All fishes, invertebrates and plants have ranges and rates of adaptation to osmotic pressure; it is necessary to be aware and accommodate all that you intend to keep together.

Temperature should range as per tropical marines, in the mid seventies to low eighties degree F., and pH elevated, and stable in the upper sevens to low eights.

Lighting is largely a matter of personal preference, unless you're utilizing live rock, plants, or other photosynthetic organisms. For beauty's sake, brighter is better.


A simple marine "fish-only" arrangement with an undergravel filter augmented by an outside power unit will do. Better are set-ups that utilize more sophisticated modes, such as live-rock and skimmer "Berlin" methods, in-tank nitrate reducing contrivances... Best is a remoted "refugium" filter/manipulation tank arrayed in tandem with the main display unit where filtration and more can be accomplished without disturbing the principal tank at all.

Brackish and seawater holds less dissolved gas than fresh, hence a plug for the use of a redundant mechanical aerator in addition to other forms of agitation.


The regular, ongoing upkeep of your system is THE key to your success. Weekly checking and adjusting of water quality is de rigueur. Get and use a GOOD hydrometer; the relatively non-breakable plastic box-type are my preference.

Pre-mixing your make-up water in a suitable container for proper specific gravity, pH and hardness (all automatic with a good synthetic salt mix) is definitely a good idea. A clean, dedicated plastic trash can or alternate container located next to the system makes changes a breeze.



Unlike many other fishes promoted as "brackish", e.g. Scats, Datnoides, some of the Puffers, Targetfishes..., the monodactylids shy on the side of being non-quarrelsome. Though not outright peaceful, most specimens will not harass other "par" marine fishes or invertebrates.

Scatophagus argus

Coius (Datnoides) quadrifasciatus

Toxotes, an Archerfish

Monos can be territorially aggressive amongst themselves, particularly in small system volumes, or where a new individual is added to an established community; therefore the suggestion of introducing all your Monos in one throw and maintaining them in good sized schools if room permits. Four or more individuals are preferable to a smaller number where one or more are bullied more or less constantly. Observe your livestock. 


This can be an area fraught with danger. Many Monos are unwittingly killed by improper or rushed acclimation. Water conditions, in particular temperature, specific gravity and pH should be matched as close as practical between the source and your intermediate/holding system. The new specimens should be left in the dark for a day, and over a period of weeks brought into approximately the same water quality as the dealer/source.

Predator/Prey Relations

For the most part, monodactylids leave other organisms be and are too fast and agile to be caught by any but the most determined large piscivore.

Reproduction, Sexual Differentiation/Growing Your Own:

Monos cannot be easily distinguished sexually; they are egg dispersers with planktonic young. Commercial production incorporates hormonal and/or environmental manipulation, hatching and rearing the fry in saline water fed on screened live algal and crustacean matter.

Feeding/Foods/Nutrition: Types, Frequency, Amount, Wastes

Monos are voracious feeders that require large, frequent meaty and vegetable meals. Live foods are eagerly accepted, and they are easily trained onto prepared mashes, dried and frozen foods.

Whatever mix of foodstuffs you use, make sure your Monos are getting their share; to the point of visible fullness. Most of the Monos I've encountered that were otherwise appropriately acquired and housed, and lost, perished due to lack of nutrition. These are immensely active, continuously curious fishes that can waste away in days if under-fed.

Disease: Infectious, Parasitic, Nutritional, Genetic, Social

Monos are remarkably tough and resistant to biological disease, when and where maintained properly. Invariably infectious and parasitic problems are linked to diminished water quality. If/when you think your fishes are coming down with "something", don't pour in a medication, do check pH, ammonia, nitrite, and specific gravity... and adjust through moderate water changes.

THE time to observe real and potential disease problems is during feeding. Watch your charges closely; are all your monos feeding? Are they interacting "normally"? A non-feeding mono is definitely cause for concern; a food-strike should be easily turned around by an offer of live food. If not, be on the look out for bullying, and get the bullied specimen(s) to a neutral corner.


Monos, beautiful, engaging, hardy, and for marine and diligent brackish water aquarists. Learn to pick out decent specimens and grant them the graces of a large, stable environment and plenty of food, and you'll enjoy them for many "moons".

Bibliography/Further Reading:

Burgess, Warren E., Axelrod, Herbert R. & Raymond E. Hunziker III, 1990. Volume 1, Marine Fishes, Atlas of Aquarium Fishes Reference Book. T.F.H. Publications, Inc. NJ.

Dawes, John, 1989. Bolstering sales of brackish water fish. Pets Supplies Marketing 7/89.

Fenner, Robert. 1997. Marvelous Monos. TFH 6/97.

Gibbs, Max, 1995. The brackish aquarium. FAMA 4/95.

Gos, Michael W., 1977. The brackish aquarium. TFH 10/77.

Gos, Michael W., 1980. The brackish system, parts 1 & 2. FAMA 11,12/80.

Nelson, Joseph, 1994. Fishes of the World. John Wiley & Sons, Inc., NY.

Palko, Barbara J., 1977. Monodactylus sebae... a single rearing attempt. TFH 7/77.

Taylor, Edward C., 1982. Keeping a brackish aquarium, parts I, II. TFH 5,6/82

Volkart, Bill, 1989. The brackish aquarium, part 3: the fishes. TFH 8/89. 

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