Can Semi-Aggressive Fish Be Kept?

Know Your Species

• Always research a fish's species before you get it. Some fish considered semi-aggressive only show aggression in very specific situations. For example, with the Siamese fighting fish or betta, only the males show any kind of aggression. On top of this, they only fight with other males -- or occasionally with similar-looking, long-finned fish. In marine aquariums, many angelfish follow a similar pattern, with males attacking other males or fish that have the misfortune of having the same color or shape.

Schooling Fish

• Most people think of schooling fish as peaceful. But some schooling fish fall into the semi-aggressive category. These fish tend to form hierarchies within their schools, with the dominant fish chasing and bothering their subordinates. If you don't have enough subordinates, these fish may pick on other species. Oddly, the solution to this problem is more fish. If you have at least six fish, these fish will usually only bother each other. Tiger barbs are probably the archetypal semi-aggressive schooling fish. With semi-aggressive schooling fish, always avoid keeping them with long-finned fish, since many of these species find long fins an irresistible target.

Breeding

• Some semi-aggressive fish have timeshare aggression. Usually docile species like freshwater angelfish and ram cichlids are typically very peaceful fish. However, when breeding, they will typically guard their nesting sites and young, chasing and attacking other fish. With fish like this, you may want to move them to another aquarium when they're breeding to avoid attacks on tank mates.

Predation

• Some fish only bother bite-size fish. Many otherwise peaceful fish will eat smaller fish if the latter has the misfortune of fitting into the larger fish's mouth. Fish like this often wind up labeled as "semi-aggressive" since they only bother smaller fish.

Reducing Problems

• You can take several steps to reduce aggression. First and foremost, always research a fish species before buying it. It's usually easier to prevent problems than to deal with them after the fact. Large tanks tend to diffuse aggression. This is particularly true of large tanks with lots of hiding places. Breaking up a fish's line-of-sight will usually reduce aggression and give territorial species room to establish territories. You can also reduce aggression by adding schools of semi-aggressive fish to the tank together. Additionally, always add the most territorial species to the tank last so they don't consider the entire tank their territory.