Even an apparently docile species can become extremely agitated when a new fish is introduced into a tank in which it has been a long-term resident. Because it is always unwise to introduce all of your marine aquarium inhabitants to a tank within a short period of time, problems of this sort of often inevitable. The lack of space and shelter will increase aggression. As competition increases, so does aggression. Therefore, if space and shelter are limited, behavioral problems usually occur.
Most reef fish use a large area of a reef for their daily activities.
It should be apparent from these figures that space limitations can lead to problems in the confines of an aquarium. Shelter sites are also important. Most fish will defend their place of refuge vigorously, particularly if shelter is in short supply. The physical characteristics of a potential aggressor's tankmates may determine how it behaves toward them. In their natural environment, reef fish do not indiscriminately attack every fish they encounter, but instead are selective about who they are aggressive toward.