Celebes Rainbowfish (Marosatherina ladigesi)

Marosatherina ladigesi, the Celebes Rainbowfish, is an elegant and captivating freshwater fish endemic to the Maros district of south Sulawesi (Celebes), Indonesia. This species belongs to the family Telmatherinidae — the sailfin silversides — a group entirely separate from the more common Melanotaenia rainbowfish despite sharing the 'rainbowfish' common name.

The Celebes Rainbowfish has a slender, elongated body with a distinctive yellow to greenish-blue lateral stripe. Its most remarkable feature is the fin structure: males develop beautiful elongated yellow and black dorsal and caudal fins with black tips, creating an elegant, sail-like appearance that inspired the 'Sailfish' common name. Females are less dramatically finned but share the attractive body colouration.

Native to the hard, clear, fast-flowing rivers and springs of Sulawesi, this species requires water chemistry that differs from most freshwater fish: slightly hard, alkaline water that reflects the island's calcium-rich geology. This makes it compatible with Sulawesi shrimp and African cichlid water chemistry rather than soft-water South American setups.

The Celebes Rainbowfish is listed as Vulnerable due to restricted range and habitat pressures, but is captive-bred for the aquarium trade. It is a peaceful, delicate schooling fish that does best in a well-maintained, stable aquarium with appropriate water chemistry and gentle companions.

Tank Setup: A minimum 80-litre aquarium for a school of eight. Provide open mid-water swimming space with moderate water flow. Plants tolerant of harder water (Vallisneria, Anubias, Java fern) along the edges. Good surface agitation. Tight-fitting lid — excellent jumpers.

Water Parameters: Temperature 24–28°C; pH 7.0–8.0; GH 8–18 °dH; KH 4–10 °dH. Prefers moderately hard, neutral to alkaline water — opposite of the soft-water blackwater setup many small fish require. Belgian tap water is often suitable without modification.

Feeding: Omnivore. Feed quality micro flake, small pellets, frozen Daphnia, baby brine shrimp, and micro worms. Feed twice daily. Small mouths — appropriate food size is important.

Tank Mates: Keep with other peaceful species that tolerate or prefer slightly harder water: other rainbowfish, larger Boraras (in appropriate water), Corydoras, Endler's Livebearers, small barbs. Avoid fin-nippers (the long fins are vulnerable), aggressive species, and very large fish.

Behaviour: Peaceful schooling fish. Males display to each other with fin-spreading, particularly the dramatic dorsal fin. Groups of 8+ show the most natural behaviour. Surface-oriented swimmers.

Health: Moderately sensitive. Susceptible to velvet and ich in poor water quality or temperature fluctuations. Avoid soft acidic water — this causes long-term health deterioration. Weekly 20–25% water changes with appropriate water chemistry.