Melanotaenia splendida is a lovely species of rainbowfish from - you guessed it - Australia. Subject to a wide range of water conditions in the wild, this is a very hardy species that does quite well in captivity. Males develop beautiful orange and bluish-green colouration with maturity. Like most other rainbows, they are best kept in mixed-sex groups, and mix well with other robust fishes such as large danios, barbs, cichlids, etc. While they're not aggressive, very shy or small fishes may be unable to compete with these fast, active swimmers.
These are juveniles around 1.5" long, just beginning to develop adult colour. While they're too small for us to confidently sex, we believe this looks like a mixed-sex batch.