Scats and Monos Old Favorites and New Species for the Brackish-Water Aquarium

Author: Neale Monks

While they are two very different types of fish morphologically, scats and monos have very similar requirements and get along together swimmingly in the aquarium.

The scats (family Scatophagidae) and monos (family Monodactylidae) are two families of perciform fishes supremely adapted to life in estuaries and other shallow, coastal waters. They are fast-moving omnivores that will eat pretty much anything they can catch, and they are remarkably tolerant of rapid changes in salinity.

In aquaria this makes them hardy and adaptable fish that settle in quickly and have proven to be durable, long-lived species ideally suited to spacious marine and brackish-water aquaria. Moreover, while scats and monos are morphologically very different, in terms of overall requirements they are very similar, and they are two types of fish that get along very well when kept together in aquaria.

Scats: Go Anywhere, Eat Anything

Of the two families, scats are perhaps the more popular. They are roughly oval in shape, with a relatively compact, pointed head ending in a relatively dainty mouth. The dorsal and anal fins are divided into small spiny regions at the front and large softer regions at the back. The soft parts of the dorsal and anal fins are large and triangular and almost run into the large and approximately triangular caudal fin. According to some accounts, the spines on the dorsal fins are slightly venomous, so it’s a good idea to handle these fish with care.

During the 1960s and 70s scats were known as the “poor man’s discus.” In comparison to the wild-caught discus available at the time, scats were hardy and easy to keep, if somewhat less colorful. Certainly, scats are more “handsome” than “pretty.” Juveniles of the most commonly traded species, Scatophagus argus, are basically bronze in color and marked with numerous small brown or black spots on the flanks. As the fish mature, they become brassier in hue and their spots are a little less distinct, particularly on the lower half of the body.

Certain populations of Scatophagus argus have distinctive variations in coloration. The redheaded scat, sometimes known by the spurious Latin name of Scatophagus rubrifrons, has red blotches along the dorsal surface of the fish, particularly on the top part of the head and below the dorsal fin. Other (invalid) names applied to this species include Scatophagus atromaculatus and Scatophagus ornatus. The Hooghly River scat is similar to normal Scatophagus argus, but the dark blotches on the flanks tend to be larger and often merge to form short bands.

There are at least three other species of scat know to science, two of which are fairly regularly traded from time to time. The most common of these is the silver scat Selenotoca multifasciata. Healthy specimens of this species are delightful fish, being bright silver in color and marked with a series of black spots and vertical bands along the body.

Much less frequently seen is the banded scat Scatophagus tetracanthus. This fish has thick brown stripes against a pale cream-colored body and is marked on the forehead with orange or red patches similar to those seen on redheaded scats. Scatophagus tetracanthus is always expensive, but despite its rarity in the hobby it is no more difficult to keep than any of the other scats.

The final species of scat currently recognized is Selenotoca papuensis, a fish that is very similar to Selenotoca multifasciata and appears not to be traded as a separate species. This isn’t to say that it isn’t sold in aquarium stores—it probably is, just under the Selenotoca multifasciata name.

Compared with Selenotoca multifasciata, Selenotoca papuensis has fewer, thicker vertical bands and the spots on the lower half of the body are much larger. Such differences are really only obvious when comparing the two species side by side, but roughly speaking, on Selenotoca papuensis the spots are about the size of the eyes, while on Selenotoca multifasciata the spots are very much smaller.

All scats are relatively large, fast-growing fish. Apart from Selenotoca papuensis, all the scats can be expected to exceed a length of 15 inches (38 cm), usually reaching at least 8 inches (20 cm) within a year or two of purchase. Obviously, such large fish will need a spacious aquarium. Selenotoca papuensis is quite a bit smaller, getting to about 3½ inches (9 cm) or so. As such, it makes a good choice for the smaller aquarium, provided you can correctly distinguish this species from Selenotoca multifasciata.

Scats are true omnivores and will eat practically any organic matter. Their Latin name, Scatophagus, literally means “dung eater,” and reflects the habit of these fish to feed on sewage, something that does much to diminish their value to local fishermen! Aquarists can feed them all the standard aquarium foods, from flakes and pellets through to bloodworms and brine shrimp. They also enjoy eating vegetable matter, whether aquarium plants or something deliberately provided for them, such as blanched lettuce. Algae of various kinds will be taken, including blanket-weed collected from ponds.

Scats are intelligent fish and quickly learn to recognize potential sources of food. This goes beyond merely learning to beg for food from their owners. I once owned an archerfish that I had trained to spit down bits of prawn from the sides of the tank above the waterline. A silver scat in the tank with the archer soon learned what was going on. It would line itself up with the archerfish and then dash forward when the archer began spitting, hoping to snap up the meal before the archer got there.

Monos: Brackish-Water Butterflies

Like Scatophagidae, Monodactylidae is a small family, with six species in two genera, Schuettea and Monodactylus. They are all active, schooling predators that eat small fish and invertebrates in the wild but happily settle for frozen foods and pellets in captivity. None of the Schuettea species are traded, but several species of Monodactylus have been used as aquarium fish.

Monodactylus argenteus is by far the most frequently encountered, and it is not only seen in domestic aquaria but also in public aquaria, where it is commonly used as an active, colorful schooling fish for use with things like sharks and groupers. It has a circular body that blends almost imperceptibly into triangular dorsal and anal fins of similar size, giving it a profile quite like an angelfish. For this reason, it is sometimes known as the Malayan angel. The dorsal fin is yellow, and it is marked with two thin black lines, one running vertically through the eye, and another somewhat obliquely past the gills, pectoral fins, and onto the leading edge of the anal fin.

Two other species are less frequently seen, Monodactylus sebae and Monodactylus kottelati. Monodactylus sebae is known as the West African mono and is immediately recognizable by its very tall body shape, being even more like a freshwater angelfish than Monodactylus argenteus. It does not have the yellow dorsal fin, but it is attractively marked nonetheless, with four vertical bands, the first two similar in location to those on Monodactylus argenteus, the third running from the tip of the dorsal to the tip of the anal, and the fourth through the base of the caudal fin.

Monodactylus kottelati has only recently been recognized as a distinct species, and importers make no attempt to distinguish it from Monodactylus argenteus. It resembles that species in shape, except that the anal fin is much deeper than the dorsal fin is tall, and instead of having a yellow dorsal fin, the dorsal fin is orange.

There is a fourth species of mono, Monodactylus falciformis, and while it can be kept in aquaria perfectly well, it does not appear to have ever been traded as an aquarium fish. Both Monodactylus kottelati and Monodactylus falciformis appear to be confined to brackish and marine waters, whereas Monodactylus argenteus and Monodactylus sebae are often found in fresh water.

Monos can get to over 12 inches (30 cm) long, but aquarium specimens are often smaller than that. A comparative dwarf is Monodactylus kottelati, which does not get any bigger than about 3 inches (8 cm), even in the wild. It is shame that this species isn’t deliberately marked out by importers and retailers; given its small size it is perhaps the most suitable of all the monos for the home aquarium. Perhaps more important than their size is their level of activity; these are quite simply among the fastest fish maintained by aquarists.

By all means put some rocks and bogwood around the edges of the tank to make these fish feel more secure, but make sure you leave plenty of open water. Any aquarist trying to net these fish will quickly learn that monos are not only fast, but they are also incredibly maneuverable, and catching these beasts is a real exercise in patience!

Maintaining Scats and Monos

Scats and monos require essentially the same things in terms of maintenance. The prime requirement is for a large aquarium with plenty of open swimming space. The smaller species, Selenotoca papuensis and Monodactylus kottelati, can probably be maintained adequately in a 55-gallon aquarium, but the other species are much bigger, and for a decent size school of the larger scats or monos, a tank around 180 gallons or so in size is necessary.

Water chemistry is important, too, and regardless of the exact salinity you aim for, you do want to be sure the pH is at least 8 and that there is a high carbonate hardness level (around 20° KH) so that the pH stays steady. Note that table salt and aquarium tonic salt won’t do anything to raise the pH and hardness level, so use a proper marine mix instead. This has lots of buffering agents that will keep the pH and hardness levels where you want them.

These are large, greedy fishes, and you need to provide sufficient filtration alongside frequent water changes. The only important difference between the two is their diet, with scats being omnivores but monos more or less entirely predatory. Since both will usually take things like flake, pellets, and frozen prawns without much coaxing, even the question of diet is easily handled.

There is much confusion over the optimal salinity for these fishes, much of it stemming from a misunderstanding of the ecology of these animals in the wild. Scats and monos are not like salmon or eels in migrating between rivers and the sea only for breeding purposes. Scats and monos of all ages routinely move in and out of estuaries while foraging for food, and they have evolved to cope with changes in ambient salinity.

There is therefore really no such thing as the perfect salinity for an aquarium containing these species. Provided the water is at least moderately brackish, scats and monos aren’t otherwise fussy, and raising or lowering the specific gravity a little with each water change will do them no harm at all.

Aiming for an average specific gravity of 1.010 is ideal, making these fish ideal companions for things like brackish-water archerfish, Colombian sharks, green chromides, large sleeper gobies, sailfin mollies, and all sorts of other medium to large brackish-water fish. All scats and monos do well in marine aquaria as well, and they make superb companions for sturdy marine fish of similar size, such as sweetlips, groupers, and lionfish.

As a general rule, scats and monos should not be kept in freshwater aquaria for long periods. This is certainly true for the species normally only found in brackish or marine conditions, such as Monodactylus kottelati. A few species, such as Monodactylus argenteus, have been kept in freshwater tanks for many years, but generally monos and scats are healthier when kept in brackish or marine aquaria.

Breeding and Behavior

Scats and monos differ somewhat in their behavior, at least in aquaria. While they appear to be sociable in the wild, monos can be a bit aggressive toward one another in captivity, at least when kept in small groups. Combining different species seems to work well, as they will form mixed-species groups in the wild, and in aquaria this gives them a sense of security without triggering any latent intraspecific aggression. Surprisingly enough, mixing monos and scats seems to work well, too.

If you want to keep just one species it is best to keep them in groups of six or more, otherwise you must be ready to remove any specimens that are being picked on excessively. Scats seem to be far mellower, and can be kept singly or in groups, either with their own kind or with other scats or monos. They are sociable, though, so you should keep at least two or three specimens.

Neither scats nor monos have bred in aquaria on a regular basis, and while reports exist for both types of fish, they are often fragmentary and usually end in failure, with the eggs becoming fungused or the fry simply dying off after a few days. It is thought that scats are substrate spawners that guard their eggs, and that monos merely scatter the eggs and display no broodcare at all, but such reports of spawning behaviors that exist are often sketchy and contradictory. About the only thing that does seem certain is that scats breed in the sea, while monos spawn in rivers. Beyond this, there is obviously ample scope for experimentation.

It appears that there are a few key problems. Firstly, while they may be maintained in brackish water, neither species will spawn in such conditions, and so they will need to be transferred either into a saltwater tank or a freshwater one if breeding is to be encouraged.

Secondly, the sexes appear to be identical (at least to the aquarist), so finding a breeding pair will inevitably involve raising a group to maturity and then isolating any pairs that form.

Finally, even if the fish can be encouraged to spawn, raising the fry will probably be just as difficult as it is for marine fish, or for that matter, freshwater fish from marine families, such as gobies and puffers. In other words, the fry will likely be small and require tiny live foods. It is quite probable that the fry will also need to be exposed to a change in salinity, mimicking the transition from fresh or marine waters where their parents spawned, to the brackish waters that juvenile scats and monos seem to prefer.

Scats and monos present few other problems. They are not particularly disease prone, and respond well to commercial whitespot remedies and other treatments, unlike certain brackish-water fish (most notably puffers and moray eels, which can be intolerant of copper-based medicines). Scats can be prone to lymphocystis and pop-eye, neither of which is fatal, but then again, neither of which is easy to treat.

Optimal water conditions are important, as is ensuring that the fish cannot damage themselves (for example against sharp rocks). Monos will turn black when stressed, and this is a good sign that something is not right, so it is usually easy to pre-empt problems when keeping these fish. Do note though, that they also turn black at night.

Otherwise, scats and monos are a joy to keep, vivacious and alert, they are truly outstanding fish for the larger brackish-water aquarium.