The Melanotaeniidae is characterised by having their distal premaxillary teeth enlarged.
They have a compressed body with the two dorsal fins being separated but with only a small gap between them.
There are 3-7 spines in the first dorsal fin while the second has 6-22 rays, with the first ray being a stout spine in some species, the anal fin has 10-30 rays and, again, the first may be a stout spine in some species.
The lateral line is either weakly developed or absent.
They have comparatively large scales and these number 28-60 in the lateral series.
The pelvic fins are attached to the fish’s abdomen by a membrane which runs along the length of the innermost ray and this is a feature which  can be used to separate rainbowfishes from silversides, although the membrane is easily torn.
The majority of the species in this family demonstrate sexual dimorphism with the males usually being the more colourful sex and also showing an elongated median fin ray.

Most species of rainbowfish are less than 12 cm (4.7 in) in length, with some species measuring less than 6 cm (2.4 in), while one species, Melanotaenia vanheurni, reaches lengths of up to 20 cm (7.9 in).
They live in a wide range of freshwater habitats, including rivers, lakes, and swamps.
Although they spawn all year round, they lay a particularly large number of eggs at the start of the local rainy season.
The eggs are attached to aquatic vegetation, and hatch seven to 18 days later.
Rainbowfish are omnivorous, feeding on small crustaceans, insect larvae, and algae.

Rainbowfish are popular aquarium fish along with Pseudomugil blue-eyes, which are another small, colourful fish found in a similar range and habitats.
In the wild, some rainbowfish populations have been severely affected by the aggressive introduced eastern mosquitofish (Gambusia holbrooki), tilapia cichlids, and pollution.
The Melanotaeniidae is characterised by having their distal premaxillary teeth enlarged. They have a compressed body with the two dorsal fins being separated but with only a small gap between them. There are 3-7 spines in the first dorsal fin while the second has 6-22 rays, with the first ray being a stout spine in some species, the anal fin has 10-30 rays and, again, the first may be a stout spine in some species. The lateral line is either weakly developed or absent. They have comparatively large scales and these number 28-60 in the lateral series. The pelvic fins are attached to the fish’s abdomen by a membrane which runs along the length of the innermost ray and this is a feature which  can be used to separate rainbowfishes from silversides, although the membrane is easily torn. The majority of the species in this family demonstrate sexual dimorphism with the males usually being the more colourful sex and also showing an elongated median fin ray.
Most species of rainbowfish are less than 12 cm (4.7 in) in length, with some species measuring less than 6 cm (2.4 in), while one species, Melanotaenia vanheurni, reaches lengths of up to 20 cm (7.9 in). They live in a wide range of freshwater habitats, including rivers, lakes, and swamps. Although they spawn all year round, they lay a particularly large number of eggs at the start of the local rainy season. The eggs are attached to aquatic vegetation, and hatch seven to 18 days later. Rainbowfish are omnivorous, feeding on small crustaceans, insect larvae, and algae.
Rainbowfish are popular aquarium fish along with Pseudomugil blue-eyes, which are another small, colourful fish found in a similar range and habitats. In the wild, some rainbowfish populations have been severely affected by the aggressive introduced eastern mosquitofish (Gambusia holbrooki), tilapia cichlids, and pollution.
Rainbowfish