Aggression in Fish

Aggression among fish, particularly marine species, is a problem that plagues both beginning and advanced aquarists. For example, you buy a new fish and add it to your apparently peaceful community tank and suddenly violence erupts. Long-term residents react aggressively toward a newly introduced fish, causing acclimation difficulties. This can lead to stress, disease and often the subsequent death of the new addition. The purpose of this article is to examine reef fish aggression and suggest techniques that will allow us to reduce conflict in our saltwater aquariums.

Provide as much aquarium space as possible for each tank inhabitant, especially if some of the species are known to be aggressive. Overcrowding your aquarium will lead to profuse amounts of fighting.

There should be a large number of hiding places in the tank. This doesn't simply mean to load the tank with coral or live rock. Make sure there are caves, crevices and holes suitable for all members of your aquarium community. Also, utilize topography to curb aggression. Using a number of smaller coral heads or groupings of live rock rather than a continuous wall of them can be advantageous in dispersing territory holders and reducing skirmishes among them. A territorial fish will often restrict its defense to one coral head, which is the core area of its territory, and the immediate area around it. If the aquascaping structure is contiguous, more battles over boundaries are likely to occur.

Avoid keeping members of the same species in the same aquarium unless they are known to school or group in captivity. Some species that school on the reef may not tolerate each other in the confines of a smaller tank. Even in schools, individuals usually maintain a specific distance between each other, and often in a home aquarium these spacing requirements cannot be met. The size of the tank is important in determining if you will be successful in keeping species members together.

If you intend to keep two fish of the same species together, it is often wise to introduce them into your aquarium at the same time.
Many of the species we keep will change sex depending on their social status. Often, a larger, more dominant individual will be one sex and the smaller, subordinate specimens in the group will be the opposite sex. To increase the likelihood of acquiring a pair in a sex-changing species, purchase two individuals of different sizes. If a subordinate individual is removed from its natural environment and isolated in a dealer's tank, it may begin to change sex. Once an individual has changed sex, it cannot be reversed. Therefore, if you place two individuals in the same tank and fighting occurs, it could be that the fish are both the dominant sex and will have to be separated. This technique is especially effective when trying to keep species such as clownfish or pygmy angelfish.

If problems do occur when you introduce a new fish, there are several techniques you can use to eliminate or minimize the hostility. Turning off all lights will temporarily reduce aggression, unless, of course, the aggressor is a nocturnal species. Fish are often less aggressive on a full stomach, so feeding your fish can sometimes be effective for short-term behavioral modification.

If these simple procedures are ineffective, then more drastic steps will be necessary. Because many territorial fish remember the boundaries of their territory by the topographical features of their habitat, changing the aquascaping around immediately before and after introducing a new fish can reduce aggression. In effect, changing the topography is the same as moving the old residents of the tank to a new area.

Another method for introducing a new fish into a more aggressive community tank is to partition a portion of the aquarium off, creating an area where you can isolate the new resident. The partition should be clear, such as plexiglass, so that the older residents can see the new fish and habituate to its presence. Depending on the type of filtration used, it may be necessary for the partition to contain numerous holes and the top to be slotted to allow water exchange between the two compartments. This isolation period allows the new fish to become familiar with a part of the tank so that it can more easily find a place to hide, if necessary, when the partition is removed. After a week or so, the partition can be removed. With any luck, aggression will be minimal.

Another option is to pull the aggressor out of the tank, isolate it in a quarantine tank and then reintroduce it after the new arrival has settled in. If all of these techniques fail, you will probably have to permanently remove the aggressive individual(s) or give up the idea of introducing more fish into the tank.