Stress is one of the worst conditions that our fish can experience as it impacts their health, growth, FCR and reproductive ability very quickly, and if left unchecked, can result in mortality.  However, there are times when we need to hold them under conditions that can cause stress, such as when a pair or small group of fish is placed in a tank to breed.  Due to the low number of fish in the tank they tend to feel vulnerable and experience this as stress.  Dither fish are fish that are introduced to a situation to act as a distraction or to focus the bond within a breeding pair.
 
An example of this is a tank of catfish breeder females that are stocked at low density to condition them for spawning, but under these conditions they are lightly stressed.  A shoal of tilapia can be added to this tank with the catfish to serve as dither fish.  Point 1 is that the water conditions both species require is the same.  The 2nd point is that the tilapia are significantly too large for the catfish to eat!  Using orange coloured tilapia is useful as this seems to further improve the calming benefit of their presence, but this is purely an observational comment.  The presence of the tilapia in mid water makes the catfish feel more secure and comfortable, lowering their stress levels, causing them to feed more effectively, and ultimately improving the quality of the conditioning.  In this instance a different species was used but the same benefit could have been obtained by increasing the density of catfish, even though they are not all required for spawning.
 
Another example where dither fish are useful is with spawning cichlids such as angelfish.  A pair is placed into a glass tank to form a bond and spawn repeatedly over an extended period of time.  In the absence of external stimuli the bond of the pair may be weak and the period for which they spawn may be reduced.  By placing a separate glass tank containing angelfish immediately adjacent to the first, the sight of the other angelfish fish causes the pair to cooperate in defending their territory against perceived rival intruders, with the result that the pair spawn more frequently and over a longer period.
 
The two examples above demonstrate that dither fish can be of the same or different species, depending on the situation and preference of the operator.  In both cases the dither fish have the impact of improving the situation for the primary fish.
 
 
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