It is essentially the rear part of the body, particularly the caudal fin, which serves to propel the fish, while the other fins play a stabilizing and steering role. Of course, the more hydrodynamic a fish's form, the more it is capable of setting off abruptly and swimming quickly, indispensable for catching prey or fleeing an enemy.

Aquarists are sometimes advised not to let quick and lively fish (like Barbs) cohabit with slower and more placid species (like loaches), as the latter may be frustrated in

their attempts to eat the food provided by their owner.


Fishes' behavior in an aquarium reflects their lifestyle in a natural habitat, albeit modified by the fact that they are living in a more cramped environment, coming into 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 the other occupants.

• 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 individuals 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 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.

• 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 capable of

A Fish with a classicalshape move forwards by propelling themselveswith the rear part of thebody.

Some fish live in groups, which makes it easier not only to defend themselvesbut also to reproduce.

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 fish from colliding with each other. »Dominance behavior

The biggest members of a species dominate the smallest ones: when the latter get bigger they are ejected from the territory. Dominance behavior has practical and social implications, as the dominator will have 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 to hide most of the time, with its growth being prejudiced as a result. This is the case with some species of African Cichlids. Prey-predatorrelationships 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 South American Cich-lids cohabit with Characins. »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 aquariums. However, a new fish introduced into a tank will often be considered as an intruder, or prey, and will be harassed.

Growth and longevity

Unlike human beings, fish continue to grow throughout their life, quickly at first, and then more slowly with age. The size of fish in aquariums is mostly smaller than that found in the wild, undoubtedly as a result of the restricted living space at their disposal. This can easily be put to the test: an individual whose size has seemingly stabilized starts to grow if it is put into a bigger tank. As regards longevity, this varies according to the species: a year, more or less, for the small species, and two to five years for the

majority of fish. Some patriarchs live to the ripe old age of ten or more - these are large fish, particularly marine species. It is very difficult to postulate an optimal life span for a given species in captivity, as environmental conditions introduce too many variable factors.


Only put together those fish which are known to be compatible, particularly in the case of marine fish, and give them as much space as possible. Be sure to provide a number of nooks and crannies, appropriate for the dimensions of the residents. Another solution is to mix species with different lifestyles - for example, free swimmers (like Barbs and Danios) and bottom-dwellers (like loaches) - that will not compete with each other.

• In nature, the biggest fish often feed on smaller ones - obviously something to avoid in an aquarium.

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