Melanotaenia affinis

Papua New Guinea.

Occurs in various biotopes, including swamps, pools and lagoons, but is most commonly found inhabiting clear rainforest streams. It tends to swim in groups around areas of dense vegetation and submerged branches. Many of these habitats are subject to seasonal changes in terms of water temperature and chemistry, making this an adaptable species.

An active species, requiring an aquarium measuring at least 120 ∗ 30 ∗ 30 cm48 ∗ 12 ∗ 12 in - ~108 litres~29 US gal.

This species, like most rainbows, looks most at home in a planted aquarium. Provide areas of dense vegetation, along with some open areas for swimming. Water movement should be quite gentle.

Temperature: 20 – 30 °C68 – 86 °F

pH: 6.5-8.0

Hardness: 8-20°H

Unfussy and will accept most dried, frozen and live foods. Regular feedings of the latter will ensure the best colouration.

Very peaceful but can disturb smaller or slow-moving fish with its rapid movements and relatively large size. Good tankmates include other similarly-sized rainbowfish, characins, danios, barbs, and catfish such as Corydoras.
The New Guinea rainbowfish is quite skittish and does far better when kept in a shoal of at least 6-8 fish, preferably more. The males will also be encouraged to display their best colours in the company of conspecifics. Obviously a suitably-sized aquarium would be required for a very large group.

Mature males are larger and exhibit brighter colouration than females. They also develop a much deeper body than females and possess elongated first dorsal and anal fins.

Breeds in a similar fashion to other Melanotaenia species. Egg scatterer. The breeding aquarium should be at least 30" long. A pH of around neutral should be fine, with a temperature of 24 – 26.5 °C75 – 80 °F. A small air-powered filter will provide sufficient oxygenation and flow. The tank should be filled with fine-leaved plants such as java moss, or nylon spawning mops. No substrate is necessary.

The adult fish are best conditioned as a group in a separate aquarium with plenty of live and frozen foods. Select the fattest pair for spawning and introduce them to the spawning tank. A small raise in temperature can induce spawning. The pair will spawn for a period of several weeks, laying a few eggs each day. These are attached to surfaces by a small thread. Although the adults tend not to eat the spawn, it's easier to raise the fry in a separate aquarium. We recommend checking the plants or mops daily and removing any eggs you find to a raising tank, containing water from the spawning tank.

The eggs hatch in 7-12 days, depending on temperature, and the tiny fry initially require infusoria-type food, before graduating onto microworm or brine shrimp nauplii after a week or so.

M. affinis was one of the first rainbowfish species to be imported for the hobby in any numbers. There are several geographical variants, three of which can be found in the hobby. The "standard" form has a large natural range and is the most commonly encountered. The "Pagwi" form has a very limited distribution, being known only from a tributary of the Sepik River in New Guinea, while the "Bluewater Creek" form has only been collected from a single location close to Madang, in Papua New Guinea. The different forms look superficially similar and are distinguishable only by small variations in colour and patterning.