Many types of fish are quite territorial and will not tolerate others of their own species or, in some cases, of other species, in close proximity. This is important for you to understand from the start so that you can be aware of potential aggressive behavior or conflicts when choosing animals for your collection.
Viewing aquatic environments in nature or on television may give you the false impression that a large variety of creatures can easily coexist in an aquarium. While it is possible to create complex community aquariums, such an undertaking requires a good deal of experience. Oftentimes, species that might coexist quite well in nature will not do so within the confines of an aquarium.
As always, it is important to learn as much as you can about your pets if you are to be successful in keeping them. Understanding a fish's natural behaviors and habitat will help you to determine whether or not it will coexist with others. In many cases, territorial behavior is engaged in to protect a favorite food source. This phenomenon may easily be observed by those keeping marine damselfish. These tiny yet aggressive beasts tend and protect small patches of their favorite algae. Despite their small size, they will make life miserable for even much larger fish that dare intrude.
Aggressive territorial behavior may be inhibited in aquariums that are too small for the establishment of territories. Keep in mind that if the fish are moved to a larger aquarium, or if the size of the available territory is increased as a result of the death of a tank mate, aggression may develop among fish that had previously cohabited peacefully.
Other common reasons for aggressive behavior among fish are the protection of a nest site or competition for mates. It is important to bear in mind that normally peaceable fish may undergo an overnight transformation in behavior once they enter breeding condition or upon the introduction of a member of the opposite sex. Review the section on mating strategies in Chapter 20 to learn more about the effects of breeding behavior upon overall lifestyle.