Australia and New Guinea Rainbowfish Creek Biotope

A creek biotope for Melanotaenia and Pseudomugil: seasonal wet and dry flow, the wide natural range of pH and hardness, planted margins with wood and leaf litter, stocking and spawning.

Creek habitats of the region

The rainbowfish family Melanotaeniidae is native to northern and eastern Australia, New Guinea and nearby islands, with representatives in Sulawesi and Madagascar, and lives in rivers, lakes and swamps (Wikipedia). The genus Melanotaenia alone contains 89 recognised species and, as presently defined, is probably not monophyletic (Wikipedia). Blue-eyes of the genus Pseudomugil, 17 species endemic to Australia and New Guinea, occupy freshwater rivers and streams as well as brackish water (Wikipedia). The creek biotope described here is the small, vegetated, slow to moderately flowing stream that most of these species share, rather than the large lakes that hold a few well-known species.

Seriously Fish describes Melanotaenia splendida splendida and Melanotaenia trifasciata as inhabitants of swamps, lagoons, streams and rivers that shelter under aquatic vegetation and submerged logs, with M. trifasciata favouring flowing streams and tributaries. Melanotaenia praecox lives in swiftly flowing tributaries of the Mamberamo river system in New Guinea and in the adjacent swamps, gathering around plants and wood. Pseudomugil gertrudae occupies the shallow, heavily vegetated margins of creeks, billabongs and swamps with dense macrophytes, submerged wood and leaf litter, while Pseudomugil signifer is usually found within 15–20 km of the sea, from mangrove creeks to forest streams. FishBase notes that M. splendida forms large schools at the surface.

Seasonal flow and chemistry variance

These creeks follow a wet and dry rhythm. Seriously Fish records seasonal fluctuations in temperature, water level and sediment load in the habitats of M. trifasciata, and Wikipedia states that rainbowfishes spawn prolifically at the start of the local rainy season. Chemistry varies more between localities than in most biotopes. FishBase reports native waters of M. splendida from 20 to 29 °C and pH 5.3 to 8.5, clear to turbid. For P. gertrudae, Seriously Fish cites wild pH from 3.68 to 9.4, conductivity of 12–646 µS/cm and temperatures of 12–34 °C; some sites are acidic and tannin-stained, others clear. P. signifer occurs naturally at pH 5.5–7.8. In contrast, the lakes of the Vogelkop Peninsula that hold Melanotaenia boesemani run at pH 8.0–9.0, while their tributaries measure only 6.0–6.5. A creek tank therefore reproduces one locality, not the whole region.

Published aquarium ranges

The ranges below are the aquarium recommendations from Seriously Fish species profiles; FishBase gives pH 6.0–8.0, 10–30 dH and 20–30 °C for M. splendida.

Melanotaenia splendida splendida: pH 5.0–8.0 (aim near neutral), 5–20 dGH, natural 20–30 °C.
Melanotaenia trifasciata: pH 6.5–8.0, typically above 7.2; 8–20 dGH; natural 24–33 °C.
Melanotaenia praecox: 23–28 °C, pH 6.8–7.5, 5–15 dGH.
Pseudomugil signifer: 20–26 °C (wild 15–28 °C), pH 6.5–7.5, 5–15 dGH.
Pseudomugil gertrudae: 21–28 °C, pH 4.5–7.5, 5–12 dGH.

Planted margins and hardscape

The layout mirrors a creek bank: dense stands of plants along the sides and back, open water in the centre for schooling and display, submerged wood and branches, and a layer of leaf litter in the calmer corners where P. gertrudae feeds. Seriously Fish calls for a planted aquarium with areas of dense vegetation and open swimming space for all the Melanotaenia listed, floating plants to diffuse light for M. praecox, and driftwood, floating plants and fine-leaved moss such as Taxiphyllum for P. signifer. Fine-leaved plants double as spawning sites: FishBase notes that M. splendida attaches its eggs to vegetation and to submerged Melaleuca roots along river banks. Water movement should be slow to moderate for the Melanotaenia species; for blue-eyes the water should be well oxygenated with a degree of flow.

Fauna and stocking

Rainbowfishes are compressed-bodied fish with two dorsal fins separated by a small gap; most species stay under 12 cm, one reaches 20 cm (Wikipedia). Melanotaenia are peaceful but skittish and fast, and Seriously Fish recommends shoals of at least 6–8, in tanks with a base of 120 × 30 cm for M. splendida and M. trifasciata and 60 × 30 cm for the smaller M. praecox. Blue-eyes are peaceful and easily outcompeted; both P. signifer and P. gertrudae are kept in groups of 8–10 or more, and P. gertrudae requires a biologically mature tank because it is sensitive to swings in water chemistry. Companions named by Seriously Fish for the Melanotaenia species include similarly sized rainbowfishes, characins, danios, barbs, freshwater gobies and Corydoras; for blue-eyes, small cyprinids, gobies and smaller rainbowfishes.

Feeding and spawning

Rainbowfishes are generalised omnivores that take small crustaceans, insect larvae and algae in nature (Wikipedia); in the aquarium they accept dried, frozen and live foods, with live foods improving colour. Blue-eyes feed on zooplankton and phytoplankton and need small live foods or finely crushed dried food. All the species here are egg scatterers that spawn over days or weeks onto plants or spawning mops. Wikipedia gives 7–18 days to hatching for the family; Seriously Fish cites 7–12 days for M. splendida and M. trifasciata at pH about 7.5 and 26.5–29 °C, 7–10 days for M. praecox, 10–21 days for P. signifer depending on temperature, and about 10 days for P. gertrudae. Fry take infusoria or Paramecium first, then Artemia nauplii.