Aquarium fish aggression
Aggression can be a serious problem in an aquarium. Typically, this is damage caused either directly during the attack, or in the collision with objects of internal design or equipment of the aquarium during the flight. Both injuries and stress can lead to fish death. Sometimes you may notice that a particular fish behaves aggressively, but only the results of aggression (injury) are visible. In the latter case, close monitoring of the aquarium is necessary to establish which particular fish (or fish) is to blame for this.
It rarely happens that some fish attack another without any reason. From the point of view of the aquarist, the attack may seem unprovoked, but in fact he simply could not understand the motives underlying this behavior.
Territoriality and territory protection
Some fish establish territory for feeding and spawning. The spawning area can be intended either simply for spawning, or (for species caring for offspring) this is sometimes quite a substantial area, playing the role of a "manger" for fry, where all other fish are strictly not allowed. Some types of harassment described above also have a territorial background. Aquarists usually view this behavior as aggressive, although in reality it is more likely defensive behavior.
This “defense attack” is usually preceded by threatening behavior. Under natural conditions, this is usually enough to warn the “uninvited guest,” for whom flight would be a natural response to such a warning. A defending fish usually does not pursue an alien outside its territory - otherwise it would have to leave its partner (partner) unprotected, or its territory, or caviar, or fry (depending on what it protects). Such defensive behavior causes problems only in an aquarium, because there is not enough large free space and the alien is not able to swim away from the attacking fish, which for this reason continues to attack him incessantly. Fish, caring for offspring, often make almost suicidal attacks on larger enemies, including such as nets, siphon tubes and the hands of the aquarist.
Sometimes there are real fights - when a fish fights for a partner (partner) or for the territory. Such fights can be quite serious. And again, for the loser, the usual reaction is to flee. Again, in an aquarium, this can lead to problems, since there is not enough space there to escape from the winner, for whom the constant presence of the defeated means that he is still trying to compete with him. Another situation is when the territories of two fish have a common border. However, most often these are ritual fights between equal individuals having the same motives for rivalry. Serious fights occur when the boundaries of territories are established for the first time.
When a new fish is launched into an aquarium where there are already fish of “territorial” species, it usually becomes an object of attack. As a rule, this confuses the aquarist, especially if he bought a new fish as a spawning partner for the fish of the same species already in the aquarium, and therefore he expected this new fish to become a welcome neighbor. However, a territorial fish sees an intruder as a newcomer, unless the new fish is ready to immediately participate in spawning. In some groups of fish, the formation of territories and the formation of pairs for spawning are interrelated processes, and the formation of pairs often precedes the “cutting of plots”. Otherwise, it is necessary that the partner, while not having his own territory, approach the partner who already has a site, giving the correct courtship signals. But this does not happen when a new fish, embarrassed by an unfamiliar environment and experiencing stress after transportation, is unceremoniously thrown into the territory already belonging to another fish.
Another seemingly inexplicable form of aggression can be observed when, after spawning, one of the partners attacks the other. In nature, this happens due to the fact that this other partner should not remain in the spawning territory (as, for example, in gourami and cockerels, as well as in cichlids incubating caviar in the mouth). But this problem can also arise when, under normal circumstances, both parents would have to protect the offspring together, and there are no other fish nearby. It is believed that this is due to the fact that the aggressive partner has a very strong instinct for protecting the territory and, in the absence of real enemies, he instead attacks his spawning partner.
Aquarists quite often mistaken for aggression the typical behavior of predators. In fact, for a predatory fish to eat another fish is no more aggressive act than for a herbivore to eat a plant. The fault in this case lies with the aquarist: he placed incompatible fish in one aquarium.
Some fish do not eat other fish, but only damage certain parts of their body. Usually in such cases, scales and fins are most often affected. In nature, these predators usually swim from one prey to another through a space of considerable size, in which there are many fish, so the likelihood that any individual fish will be seriously damaged is very small. However, in an aquarium in a short period of time, such a predator can cause great harm. Obviously, in the aquarium, where there are other fish, such "specialized" predators simply do not belong. This type of behavior should not be confused with the behavior of fish that bite alien fins. Their victims are usually fish with veil fins, from which they bite off the tips. It is assumed that the reason is as follows: the attacking fish sees the moving tip of the fin and mistakenly takes it for something edible. Indeed, in order to catch any small live prey, a quick reaction is necessary, and the fish do not have time to stop and carefully consider what exactly it is missing.