Dither fish

Dither fish are aquarium fish used by cichlid owners to make some species of cichlids less afraid. 
Dither fish usually swim around the top of a fish tank. This behavior makes afraid fish less afraid. Fish that are not afraid hide less. The scared fish think (correctly) that no predators are in the tank. 
The technique is based on the ability of cichlids in an aquarium to use the behaviour of other fish species as a sign that the place they are in is safe. Good dither fish are usually schooling species, such as some "Danio" species, barbs and some tetra species.