• Scientific name: Melanotaenia affinis
• Synonyms: Rhombatractus affinis, Rhombosoma sepikensis
• Common name: New Guinea Rainbowfish
• Group: Rainbowfish
• Habitat: Asia; Papua New Guinea
• Size: 12 cm
• Biotope: Inhabits clear rainforest streams, but it can be found in swamps, pools and lagoons.
• Social behavior: Peaceful, active schooling fish, but can disturb smaller or slow-moving fish.
• Diet: Carnivorous; predominately live foods of all kinds, but also freeze-dried and flake foods.
• Breeding: Easy
• Tank: Minimum 110 litres
• Population: 6-8 fishes for 200 litres
• Decoration: Provide areas of dense vegetation, along the sides of the tank, with some open areas for swimming. Water movement should be gentle.
• Temperature: 20-30°C
• pH: 6.5-8
• Hardness: 8-20 NK°
• Lifespan: 5-8 years

Description: Melanotaenia affinis was one of the first rainbowfish species to be imported for the hobby in any numbers. They have an olive green body color, with a blue to blackish stripe from the snout to caudal fin base, often faint or absent in the pectoral fin region and becoming more intense and broader on the caudal peduncle where it is bordered by orange or reddish stripes. Three main varieties have been maintained in the hobby: the "standard" coloured variety is widespread in its natural habitat, the "Pagwi" variety, known only from small tributaries of the Sepik River, and the "Bluewater Creek" variety was collected from a stream near Madang. The different forms look superficially similar and are distinguishable only by small variations in colour and patterning.

Males are longer, more colorful and have pointed dorsal and anal fins. Melanotaenia affinis is closely related to Melanotaenia goldiei, but it has a less pronounced lateral stripe, and the anal fin and the second dorsal in adult males are orange or pink.

The breeding tank should be filled with fine-leaved plants such as java moss, and no substrate is necessary. Before spawning you should keep the males and females in separate aquariums for a few weeks and give them plenty of live foods. Then select the fattest pair for spawning. When the parents are well fed they ignore the young, but it's easier to raise the fry in a separate aquarium. During spawning periods approximately 10-12 eggs are deposited daily amongst dense vegetation. The eggs hatch in 7-12 days, depending on temperature, and the small fry should be fed with infusoria, and with brine shrimp nauplii after a few weeks.