Fishes at the Rainbow's End: An Introduction to Rainbowfish and Silversides

Author: Neale Monks

The peaceful and attractive rainbowfish and silversides can add beauty and interest to harder, more alkaline freshwater systems.

The Atheriniformes are a small group of bony fish that are primarily marine in distribution but do include a number of freshwater species that have become moderately important aquarium fish. There are at least eight families in the order Atheriniformes, but the only one of any great significance is Melanotaeniidae. This family includes most (but not all) of the species commonly known as rainbowfish. Although many aquarists associate these fish with Australia, they are also to be found in New Guinea as well. Actually, New Guinea is home to one of the most spectacular of all the rainbowfish, the famous red Irian rainbowfish Glossolepis incisus.

Only two other families offer up any consistently available aquarium fish, but these both give us only a single species each. From the Telmatherinidae comes the ever-popular Celebes rainbowfish Marosatherina ladigesi (in older books referred to as Telmatherina ladigesi), while a bit less frequently seen is the Madagascar rainbowfish Bedotia geayi from the family Bedotiidae. Another family of some importance in Australia is the Pseudomugilidae, or blue-eyes family, with a fair number of these lovely little fishes being popular for fresh- and brackish-water aquaria. Sadly, few if any of these fish are regularly sold in my local (British) aquarium stores.

The Atheriniformes are closely related to the Beloniformes (halfbeaks and flying fish) and the Cyprinodontiformes (killifish and livebearers), and together they form a large group known as the Atherinomorpha. As a group they can be characterized as dwellers at the surface, and most are adapted to feed on the algae, plankton, or small animals such as insects that are to be found there.

The aquarist can spot some other similarities, too. They all have a body morphology somewhere between that of primitive fish like characins and advanced fish like cichlids. Unlike characins, but like cichlids, their pelvic fins are close to the front of the fish. If you look at a primitive fish like a characin or catfish you’ll notice that the pelvic fins are set way back along the body close to the tail. The Atheriniformes in particular also share another similarity with advanced fish: they have not one, but two dorsal fins. But in other ways the Atherinomorpha are less advanced than the cichlids or any of the other perch-like fishes. They don’t have such complex mouthparts, for example, and the dorsal and anal fins lack the sturdy, defensive spines so typical of perciform fish.

The biology of many of the Atheriniformes is remarkably consistent. They are relatively small, schooling fish with little or no predisposition to being either aggressive or territorial. What makes many species even more attractive as aquarium fish is that they are often colorful, playful fishes that settle in quickly and accept a wide range of foods including flake and frozen foods.


Bedotiidae is a small family confined to Madagascar. At least nine species are described with perhaps as many again known but not officially named. However, as mentioned before, only one of them, Bedotia geayi, has any kind of presence in the hobby. This fish is commonly called the Madagascar rainbowfish and prefers neutral to moderately hard water conditions. You will sometimes see this fish recommended as a brackish-water species; it isn’t, and unless your local water is soft and acidic there’s no reason to add salt specifically for this fish.

Bedotia geayi should be an ideal community fish, but it does have some shortcomings that have limited its appeal over the years. In its favor is the fact that it is an active fast-moving fish that can make an excellent dither fish for tanks containing small to medium cichlids that aren’t aggressive or predatory. Provided the rainbowfish have swimming space, they generally stay well clear of the substrate and out of the territories of the cichlids. For their part, the cichlids appreciate the activity of the rainbowfish and more readily settle into aquarium life. Also, no one is going to tell you that the Madagascar rainbowfish isn’t a good-looking fish. Essentially a streamlined silvery fish, the males have bright orange dorsal and anal fins trimmed with black, and there is a bold purple stripe running along the flanks and onto the tail—all together making an exceptionally smart-looking fish.

But on the downside, the Madagascar rainbowfish is a fish of fast-flowing mountain streams and is most comfortable kept at a relatively low temperature compared with many other tropical fish. Somewhere around 20° to 24˚C (68° to 75°F) is best, and this is a little below the comfort zone for most cichlids. It is also intolerant of nitrite and ammonium and needs excellent filtration to do well. Ideally these fish should be kept with other species that like clean water with a strong current and only moderate temperatures.

Some good choices would be hillstream loaches, Scleromystax barbatusRhinogobius duospilus, white cloud mountain minnows, and danios. This is a nice mix of species to be sure, and could easily form a wonderful community tank, but as far as a general purpose community fish goes, the Madagascar rainbowfish should be approached with caution.

While Bedotia geayi is quite commonly traded thanks to the relative ease with which it can be bred on a commercial scale, in the wild it is threatened with extinction, just like many other Madagascan fishes. Indeed, several species of Bedotia are in trouble, being largely restricted to just a few streams or rivers. Deforestation and the conversion of land from wilderness into farmland bring about all sorts of ecological changes that these fish simply cannot adapt to. When the trees are cut down, soil washes into the streams more readily, making them much murkier and thus inhospitable to these fish that have adapted to crystal clear waters. Bedotia aren’t the only fish in trouble; the cichlids of Madagascar are in much the same pickle, and it could easily end up that only those fish that have made the jump to aquarium life have any hope of survival over the long term.

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Madagascar Rainbowfish

Latin Name: Bedotia geayi

Size: Up to 8 cm (approximately 3 inches)

Water Requirements: Hard alkaline water is best, with plenty of oxygen and a good strong current

Food: Flake, bloodworms, brine shrimp, etc.

Social Behavior: Peaceful, schooling fish

Breeding: Egglayer


Another small family, Telmatherinidae is confined to Sulawesi, New Guinea, and the islands in between. Fish in this family are sometimes called the “sailfin rainbowfish” on account of the rather large dorsal and anal fins many species possess. Most live in fresh water, but a few inhabit brackish or shallow marine environments as well. Only one species is of any importance in the hobby and it’s the Celebes rainbowfish Marosatherina ladigesi.

Marosatherina ladigesiis a very attractive species that tolerates a wider range of temperatures than Bedotia geayi and thus makes an excellent addition to a community tank containing other tropical fish. Both sexes are essentially silvery at the front and transparent behind but have edges with yellow and black and a neon-blue stripe along the flanks. Males have greatly extended fin rays and tend to be a bit more brightly colored, but even the females are pretty fish, and a school of these is a very pleasing sight indeed. Combine its looks with the fact that it does best in hard alkaline water and you have an outstanding choice for aquarists who are stuck with water that isn’t suitable for many of the more popular tetras and barbs. Marosatherina ladigesi is completely peaceful and mixes well with other placid species that enjoy the same water conditions, such as platies, guppies, bumblebee gobies, and most of the Australian rainbowfish.

The Celebes rainbowfish has a reputation for being a bit delicate. Like many fishes derived from marine ancestors (like halfbeaks, puffers, and monos, for example) it has little tolerance for rapidly changing water quality, particularly changes in pH. Adding marine salt mix helps by increasing the buffering capacity of the water, slowing down any water chemistry changes. That’s why it is often recommended that these fish be kept in slightly brackish water even though they are strictly freshwater fish in the wild.

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Celebes Rainbowfish

Latin Name: Marosatherina ladigesi, but in older books usually referred to as Telmatherina ladigesi

Size: 7 cm (approximately 2¾ inches)

Water Requirements: Hard alkaline water; salt is not essential but adding a small amount (specific gravity up to 1.003) can help in soft water areas or if the fish get fungus on their fins

Food: Flake, bloodworms, brine shrimp, etc.

Social Behavior: Peaceful, schooling fish

Breeding: Egglayer


This family is commonly called the “blue-eyes,” and it is indeed the case that many species have large, bright blue eyes. They are all comparatively small fish found in the freshwater streams of Australia and New Guinea. Unlike Bedotia geayi or Marosatherina ladigesi, many of the blue-eyes are fairly picky about water conditions and do not tolerate water chemistry changes well at all. This, coupled with the fact that none are commercially bred, has meant that on the whole they have never been very popular aquarium fish outside of Australia. This is a shame, because like many of the Atheriniformes discussed here they are active, colorful fishes that can make excellent community tank residents.

About the only example that you are likely to find on sale is the Pacific blue-eye Pseudomugil signifer, a species from Queensland that is fairly tolerant and will do well in anything from slightly acidic water to slightly brackish water. Very occasionally, specimens of Pseudomugil furcatus and Pseudomugil gertrudae turn up; these are pretty little fish that have quite specific water chemistry demands. Pseudomugil furcatus needs slightly alkaline, moderately hard water, whereas Pseudomugil gertrudaewants soft, slightly acidic water.

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Pacific Blue-Eye

Latin Name: Pseudomugil signifier

Size: 4 cm (approximately 1½ inches)

Water Requirements: Neutral water conditions preferred, but adaptable

Food: Flake, bloodworms, brine shrimp, etc.

Social Behavior: Peaceful, schooling fish

Breeding: Egglayer


Among the Atheriniformes, this is the family of prime importance to aquarists. Over the years more species have steadily become more readily available, which is splendid news because these fish are invariably peaceful, colorful, and easy to look after. Most do best in neutral, moderately hard water conditions, but they also thrive in hard alkaline water as well, making them good fish for aquarists without access to the soft and acidic water that so many barbs, rasboras, and tetras prefer. About the only shortcoming to this group of fish is that they tend to be a little more expensive than tiger barbs or neons, but this is easily outweighed by their significantly greater hardiness, longevity, and adaptability. All in all, rainbowfish are excellent aquarium fish.

Among the most widely traded rainbows are species of the genus Melanotaenia. The red-striped rainbow Melanotaenia splendida rubrostriata is widely available and particularly easy to keep. It is a robust, fairly large (up to 5 inches long) fish that can work well with peaceful cichlids and the more boisterous livebearers, such as swordtails. On the other hand, when it is young it isn’t spectacularly colored, being basically silvery with red and blue markings along the back half of the body and on the unpaired fins. It’s only once the fish become mature—and especially when they are spawning—that the rich red markings on the flanks become fully developed.

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Red-Striped Rainbowfish

Latin Name: Melanotaenia splendida rubrostriata

Size: 12 cm (approximately 4¾ inches)

Water Requirements: Neutral to slightly alkaline conditions preferred

Food: Flake, bloodworms, brine shrimp, etc.

Social Behavior: Peaceful, schooling fish

Breeding: Egglayer

Another fish that is easily overlooked when small is Melanotaenia boesemani. This fish is similar in size to the red-striped rainbow but is divided into a steel-blue front half and orange-yellow back half. Males are more brightly colored than females, but to get the males to color up nicely you do want to make sure that there are some females in the tank. As with many of the rainbowfish, male Melanotaenia boesemanispend quite a lot of time flicking their fins and displaying to one another. I’ve even seen sailfin mollies and rainbows display to one another, though I’m sure the molly’s huge dorsal fin must have given the rainbowfish an inferiority complex!

A third species of Melanotaenia that has rightly become very popular is the dwarf neon rainbow Melanotaenia praecox. A small (about 2½ inches long), hardy, and inexpensive species, this is among the very best acquisitions for anyone new to these fish. It is very pretty, having a metallic sheen that glistens red, purple, or blue (depending on the lighting). Males have fins with red edges, while females have orange markings. As with all rainbowfish, the more fish you get, the better the school will look.

Increasingly sold is the threadfin rainbowfish Iriatherina werneri. Another small species, this fish has a very streamlined appearance and a decidedly pointed snout. But the things that really make this fish stand out are the male’s extraordinarily well-developed dorsal, anal, and tail fins. The first dorsal is tall but rounded and usually a smoky yellow or orange, while the second dorsal and the anal fin are essentially transparent at the base, turning black toward the tips. They aren’t quite as easy to keep as some of the other rainbows, needing moderately hard neutral water to do best, but are otherwise not problematical.


The Atheriniformes are undervalued but very useful fish that include many species that do exceptionally well in the hard, alkaline water that many aquarists have to put up with. They are invariably peaceful schooling fish that readily take to all the usual aquarium foods. And since they are friendly and colorful they make wonderful community tank residents. Let’s hope that over the next few years importers and retailers will bring many more of these lovely fish into the hobby!