Melanotaenia lacustris

Originating from Papua New Guinea, this active and social species is a vibrant schooling fish that thrives in groups and requires a spacious aquarium setup. As a fast-swimming omnivore, it is not a fussy eater and will readily accept a variety of prepared foods like flakes and pellets, as well as frozen or live options. These fish prefer to occupy the middle and upper levels of the water column, so it's important to provide plenty of open swimming space. Due to their active nature and schooling requirements, they should always be kept in a group to ensure they feel secure and display their natural behaviors. Keeping them singly or in small numbers can lead to stress. A well-maintained tank with good water movement is essential for their health. Their peaceful disposition makes them a good candidate for a community aquarium with other similarly sized and active fish. This species is recommended for hobbyists who can provide a large environment to accommodate their energetic lifestyle and social needs.

This species is endemic to a single lake in Papua New Guinea, a unique environment characterized by clear, alkaline, and moderately hard water with some current. Replicating these conditions is key to their long-term health in an aquarium. A robust filtration system that creates moderate water flow is highly recommended, as it emulates the oxygen-rich currents of their natural habitat and helps process the waste from these active fish. Their diet in the wild is omnivorous, consisting of small crustaceans, insect larvae, and plant matter. In captivity, they thrive on a varied diet that includes a high-quality staple flake or pellet, supplemented regularly with frozen and live foods to ensure they receive a full range of nutrients. Behaviorally, this is a quintessential schooling fish that exhibits complex social interactions within its group. They are very active and require a large tank with substantial open space in the middle and top levels for swimming. A minimum school size is necessary to prevent stress and encourage natural shoaling behavior, which is fascinating to observe. While they need open water, a well-planted perimeter with hardy plants can provide security and places for retreat without impeding their swimming area. They are generally peaceful towards other species but their constant activity might intimidate slow-moving or timid tank mates. Ideal companions would be other robust and active fish of similar size that appreciate the same water conditions. Due to their specific needs for space and water chemistry, they are best suited for intermediate aquarists.

Melanotaenia lacustris is a member of the family Melanotaeniidae, a group commonly referred to as rainbowfishes. The specific epithet, lacustris, is Latin for 'of the lake,' a direct reference to its highly restricted and endemic distribution within Lake Kutubu in Papua New Guinea. Morphologically, this species displays a classic compressiform (laterally compressed) body shape, which is an adaptation for agile maneuvering and sustained swimming in a pelagic environment. This body plan allows for rapid bursts of speed and efficient movement through the water column. Physiologically, M. lacustris is adapted to the specific limnological conditions of its native habitat: alkaline pH and relatively high water hardness and total dissolved solids. Its metabolism, oxygen consumption, and waste production are categorized as medium, which is typical for an active, medium-sized fish within the order Atheriniformes. Ecologically, it fills a niche as a pelagic omnivore, feeding on zooplankton, small invertebrates, and phytoplankton. The most critical aspect of M. lacustris is its conservation status, which is listed as Vulnerable by the IUCN. Its population in the wild is threatened by a combination of factors, including the introduction of non-native species into Lake Kutubu and potential environmental degradation from local human activity. This has led to a decline in its natural population, making it a species of significant conservation concern. Consequently, established and genetically diverse captive populations are vital for ex-situ conservation and serve as a crucial safeguard against its potential extinction in the wild.

Breeding this species is considered moderately challenging but achievable for a dedicated hobbyist. To encourage spawning, it is best to set up a dedicated breeding aquarium. A recommended ratio for a breeding group is two or three females for every one male, which helps to disperse the male's persistent spawning attention and reduce stress on the females. Distinguishing between the sexes is generally straightforward in mature individuals. Males are typically larger, have a deeper body profile, and develop more extended and pointed posterior rays on their dorsal and anal fins compared to the smaller, more plainly rounded females. This is an egg-scattering species that does not exhibit parental care. Spawning typically occurs in the morning, with the female depositing adhesive eggs daily over several days onto fine-leaved plants or artificial spawning mops. To maximize the yield of fry, the spawning media containing the eggs should be moved to a separate rearing tank, or the parents should be removed from the breeding tank after spawning is complete to prevent them from consuming the eggs. The eggs will hatch after approximately one to two weeks, depending on water temperature. The newly hatched fry are minuscule and require microscopic foods such as infusoria, liquid fry food, or other specialized preparations for the first few days. As they grow, their diet can be gradually shifted to slightly larger foods like newly hatched brine shrimp and finely crushed flake food. Gentle filtration, such as a sponge filter, is essential in the rearing tank to provide oxygenation and filtration without creating a current strong enough to harm the tiny, free-swimming fry.