Sky fish

Sky fish were fish-like creatures native to Ember. They were able to swim via electricity in the crystalline fog of the planet. The fish were attracted to music, as it resonated the crystals in the fog and caused pleasing delta waves.

There were many species of sky fish, ranging from finger-sized, guppy-like creatures to giant sky sharks. (DW: A Christmas Carol)

Behind the scenes
According to The Brilliant Book 2012, a non-canonical REF book; sky fish were plucked from the skies to be sold on the Scratchington Fish Market.