harlequin fish 
noun
A small brightly coloured freshwater fish of SE Asia, popular in aquaria.
- Rasbora heteromorpha, family Cyprinidae.
- Probably the most popular rasbora, harlequin fish are lively and peaceful, and are best when swimming in a shoal.
- One large tank contains mostly smallish goldfish, and the other a mixture of tropical fish, including angels, kribs, red-tailed sharks, Chinese algae eaters, harlequin fish, various tetras, and corys.