Rheocles is a genus of small rainbowfish in the family Bedotiidae, endemic to the island of Madagascar. The atlas covers 7 species ranging from about 1.5 in up to 5.5 in, occupying streams, rivers, and a few lake systems across the island. Like other Malagasy rainbowfish, they are slender-bodied, schooling fish with two dorsal fins and often subtle, iridescent colouration rather than the bold reds and blues of their Australasian rainbowfish relatives. Madagascar's freshwater fish fauna is highly endemic and isolated, and the genus's entire natural range sits within the island's river and lake systems.
Every one of the 7 species the atlas tracks for this genus is flagged under the CARES conservation program, an unusually complete conservation-priority listing that reflects just how precarious Madagascar's endemic freshwater fish fauna has become; the IUCN tally here runs from Data Deficient through Endangered to Critically Endangered, with no species rated Least Concern. Katrana, Rheocles alaotrensis, from Lake Alaotra, is the best-known case: once an abundant lake fish, it has been squeezed by introduced predatory and competing fish alongside wetland conversion around the lake. In the hobby, Rheocles is a genus for dedicated Malagasy-endemic specialists rather than the general trade, where interest centres as much on ex-situ conservation breeding as on ordinary fishkeeping.