It can have a couple of different meanings depending on your point of view. Some would say dither fish are there to make your "prize" fish more comfortable because it see's the dithers and becomes more relaxed. Dithers can also accentuate the "prize" fish in question by creating contrasting colours or size. Thirdly I'd say dithers are also used as targets for aggressive fish, to spread out aggression or draw it away from another fish.
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