A lively, energetic fish famous for its petal-shaped tail.
Long ago, there was a funny story that said that you could get Aizen Medaka by throwing normal Medaka into a pot of blue dye. This was most likely born from a child's over-active imagination, but has nonetheless given rise to not a few unscrupulous fish hawkers. This in turn has given rise to some conversations that go something like "Hey, is it just me, or is the blue on your fish washing off?" In response, the Ministry of Civil Affairs specifically posted this notice: "The market is a wet and wild place. Beware of influxes of blue fish."