Taxonomy & naming
Bedotia geayi was described by the French ichthyologist Jacques Pellegrin in 1907. It belongs to the family Bedotiidae, the Madagascar rainbowfishes, a small endemic radiation placed in the order Atheriniformes alongside the silversides and the New Guinea–Australian rainbowfishes of the family Melanotaeniidae. Eschmeyer's Catalog of Fishes records the valid name as Bedotia geayi Pellegrin, 1907; because the species was described in Bedotia, where it remains, no parenthetical authority is used.
The genus name honours Maurice Bedot (1859–1927), the Swiss zoologist and hydrozoan systematist who directed the Geneva natural history museum, while the specific epithet geayi commemorates Martin-François Geay (1859–1910), a French pharmacist and natural-history collector who gathered specimens in Madagascar. The genus Bedotia holds a number of similar, narrowly endemic stream fishes that are hard to tell apart on body shape alone; colour pattern and locality do most of the separating. That has caused lasting confusion in the aquarium trade, where the name Bedotia geayi has often been pinned on fish that are actually Bedotia madagascariensis. Much of the older hobby literature labelled 'Madagascar rainbow' as B. geayi may in fact describe its commoner relative.
Morphology
Bedotia geayi is a small, elongate, laterally compressed fish reaching about 3.5 in in total length. The body is slender and silvery in the manner of the silversides, carrying the divided dorsal fin characteristic of the rainbowfish lineage — a short anterior dorsal followed by a longer posterior dorsal — together with a separate anal fin.
The colour pattern is the most reliable field character. Males carry two dark stripes running parallel to the lateral line: an upper stripe extending from the mouth through the eye to the base of the tail, and a lower stripe running from the throat to the base of the anal fin. The caudal fin is the giveaway — edged with a black crescent that leaves the lobes glowing bright red. The split dorsal and anal fins are edged in black and red, the rear portions taking on an iridescent gold-orange sheen in well-conditioned, displaying males. Females are far plainer: the dark markings are there but the fin colour is not, and the tail shows only a slight red edge. Mature pairs are easy to tell apart.
Habitat
The species is endemic to the Mananjary River basin of eastern Madagascar. It is a fish of small streams, favouring the well-shaded, quieter sections at moderate altitude — recorded between roughly 300 and 2133 ft elevation. Juveniles keep to the shallows while adults move into deeper water, and the fish travel in loose shoals of up to about a dozen, sorted roughly by size.
It is absent from low-altitude habitats within the Mananjary basin. That gap is blamed on deforestation in the catchment, particularly near the town of Mananjary, which has increased siltation and raised water temperatures in the lower reaches past what the species will tolerate. Water chemistry reported from the genus's clear forest streams runs neutral to slightly alkaline and moderately hard, at cool-to-warm tropical temperatures. The fish lives where it does — shaded upland creeks — and that confinement is why its range is both small and shrinking.
Feeding
Bedotia geayi is a micropredator and opportunistic omnivore with a low trophic level, feeding chiefly on small invertebrates taken from the water column and the surface. In its forest streams the diet centres on aquatic and terrestrial insects and other small invertebrates that drift or fall into the shaded water.
In the aquarium it is a greedy feeder that takes a wide range of foods — good-quality flake, plus frozen and live items such as brine shrimp and white worms. It is easy to overfeed, so small, frequent meals suit it better than large ones, and a varied diet with a live or frozen component keeps both colour and breeding condition up.
Mating
Like other Madagascar rainbowfishes, Bedotia geayi is an egg-scattering, plant-spawning species with no parental care, and it spawns more or less continuously under good conditions rather than in a single seasonal burst. Males are at their most colourful when displaying, flushing the red tail and gold-orange fins to full intensity. Competition sharpens it, so a group displays more vividly than an isolated pair.
A receptive female follows a displaying male into fine-leaved vegetation, where the pair release a few eggs at a time. Since the fish is a trickle spawner — a handful of eggs a day rather than one clutch — spawning is drawn out across many days.
Breeding
Bedotia geayi has a reputation as one of the easier rainbowfishes to breed and is often recommended as a good first breeding project. Conditioned adults spawn onto fine-leaved plants or synthetic spawning mops, scattering eggs that are large for a rainbowfish — around 0.5 in in diameter — at a rate of only about five to ten per day in a continuous trickle.
Development is easy to follow: the embryo's eyes show within two to three days, and fry hatch after roughly nine. The newly hatched fry are large and robust, able to take brine shrimp nauplii straight away, which makes them comparatively easy to raise. The catch is the adults — they eat both eggs and small fry, and reports note them killing conspecific juveniles over about 0.5 in — so eggs or mops have to go into a separate rearing container. Keepers report success with large, regular water changes, on the order of half the volume every ten days or so, which fits the species' sensitivity to declining water quality.
In the aquarium
An active, peaceful schooling fish, Bedotia geayi belongs in a group — that is where males keep their colour, displaying against one another. It is a restless, surface-oriented swimmer out of flowing streams, so it wants some water movement, clean well-oxygenated conditions, and an open swimming lane, with planted margins and spawning sites for cover and breeding.
Water quality is where it can be fussy, and it rewards stable, well-maintained conditions: keepers do well with neutral-to-alkaline, moderately hard water and regular large water changes, and FishBase lists a tolerated range around pH 7.0–8.0 at cool-to-warm tropical temperatures. It eats anything but is prone to overfeeding, so measured feeding and good filtration matter. Give it clean, moving water and the company of its own kind and it makes a rewarding community fish.
Conservation
Bedotia geayi is assessed as Endangered (EN) on the IUCN Red List under criteria B1ab(i,ii,iii,iv,v), in an assessment dated 21 June 2016. The listing reflects a narrowly restricted range — a single river basin in eastern Madagascar — combined with documented and ongoing habitat decline.
The main threat is degradation of its stream habitat through deforestation in the Mananjary catchment, which increases siltation and raises water temperatures — already enough to push the fish out of lower-altitude reaches it would otherwise occupy. A single-basin endemic confined to shaded upland creeks has little to fall back on: there is no large outlying population to buffer local losses. Its long history in the hobby and its ease of captive breeding keep it in tanks well beyond Madagascar, but that does nothing for the wild habitat, and the wild population's future rides on what happens to the forested headwaters of the Mananjary basin.