Another article so people will know better why fish behave differently. Behavior
Fishes' behavior in an aquarium reflects their lifestyle in a narutal habitat, albeit modified by the fact that they are living in a more cramped environment, coming from contact with other species more quickly and easily. Fish from the same species can behave differently from one aquarium to another, according to the capacity and other oocupants. Territorial behavior
When fish are in their original biotope, their territorial behavior is reproduced in captivity, and is sometimes even intensified. A territory is a living space-either permanent or temporary(as in the reproduction period)-with an extension proportional to the size of the fish. Its occupant rebuffs inidividuals from the same species, from related species, or even from totally different ones. The surface area must be sufficient for the fish to find refuge, foodstuffs, and the fish of the opposite sex with which to reproduce. With some fish, particularly marine species, it is important to plan a territory in the aquarium that will provide shelters and hideaways.
~Two female blue rams intimidating each other in their attempts to defend their own territory. Group behavior
Strength is to be found in unity, and living in a group permits a better defense against enemies. Indeed, from a distance a group or school of fish takes on the appearance of a mass that is capble of surprising and intimidating an enemy. Group life also facilitates reproduction, as an individual has a greater chance of finding a fish of the opposite sex. A group's unity and organization are governed by a series of signals which are invisible to human eyes; the use of the lateral line, for example, prevents the fish from colliding with each other. Dominance behavior
The biggest members of a species dominate the smallest ones: when the latter gets bigger they are ejected from the territory. Dominance behavior has practical and social implications, as the dominator will have a priority in food and the choice of a fish of the opposite sex. At the bottom of the social ladder, the most dominated fish is permanently subject to aggression and harassment and has or hide most of the time, with its growth being prejudiced as a result. This is the case with some species of African cichlids. Prey-predator relationships
Some fish feed on other smaller ones in a natural habitat, giving rise to incompatibilities in an aquarium: take care, for example, not to let some South American cichlids cohabit with Characins.
Another example is the case of raising siblings of pike cichlids. Most of the time, the biggest ones will have a tendency to cannibalize their smaller siblings in which case separation is recommended.
~A picture showing a Pearl Gourami
on the arowana's mouth. Anything that fits in their mouth would mean predation. Aggression
Sometimes an aquarium is a stage for aggression between different species. This aggression is always justified, as it is related to the defense of territory or offspring. It is a problem of space-these phenomena are rarely seen in big aquaria. However, a new fish introduced into a tank will often be considered as an intruder, or prey, and will be harassed.
The Complete Aquarium Guide by Konemann