Dear Enemy

Many territorial fishes in both freshwater and marine environments (e.g Lake Malawi Cichlids) are used to having a stormy relationship with their neighbours regarding territorial disputes and display what researchers call the ‘Dear Enemy’ phenomenon. 

This 'Dear Enemy' effect is an ethological phenomenon (that is, a behavioural trait that is observed under both natural & artificial conditions, as opposed to behaviourism, which focuses on response studies only seen in a laboratory setting) in which two neighbouring territorial animals become less aggressive toward one another once territorial borders are well-established. This means that males learn to tolerate rival males in adjacent territories and posturing 'over the garden fence' is minimised. In other words, as territory owners become accustomed to their neighbours, they expend much less time and energy on defensive behaviours directed toward one another. However, as a result of this, unfamiliar males are attacked far more often and with an increased enthusiasm! 

Additionally, as fish have no self-awareness and therefore have no idea of what they look like, this behaviour can be duplicated in an aquarium by using a mirror. A classic example of this is seen in marine fairy basslets (e.g. Royal Gramma fish). At first, the territorial aquarium residents will spend a lot of time by trying to threaten their 'neighbour' (in other words, their reflection) until they get used to this stranger who 'owns the tank next door' and accept they can’t be defeated.