Tiger barbs have unusual temperament: Unlike most schooling fish, tiger barbs are aggressive -- but unlike most fish aggression, tiger barbs' aggression is not tied to territory or predation on smaller fish. Good practices can defuse tiger barb aggression, both between other tiger barbs and other fish sharing an aquarium.
Tiger Barb Aggression
Tiger barbs typically display two types of aggression. Within their schools -- and with related barbs -- tiger barbs typically form a hierarchy. Males constantly chase and nip at each other, jockeying for position within their pecking order. This kind of behavior gets more intense the smaller the group is. Tiger barbs kept in too small a group may kill each other and are more likely to attack other species.
Tiger barbs, like most schooling fish, hang out in large groups for protection in the wild. In the confines of an aquarium, they will get stressed if you keep them in groups of fewer than eight to 10 fish. Stressed fish succumb to disease more easily than health fish. Tiger barbs' large schools have another purpose. In a large school, the aggression of the "higher ranking" fish will get dispersed among the subordinates, making it less likely they will get seriously harmed by the dominant fish.
Building Your School
You have several options when adding tiger barbs to your aquariums. Ideally, you should add them as a group. This makes it less likely that any fish has an edge over the others when they're forming their hierarchy. Selective breeding has also produced several wildly different variations on the original tiger barb coloration, including pale albinos and greenish tiger barbs. Different color varieties of tiger barbs will readily school together, making a more dynamic school.
Tiger Barbs and Other Fish
Tiger barbs rarely kill outright. However, their aggressive nature may lead indirectly to the death of tank mates. Tiger barbs are fast-swimming, manic fish; slower, shyer tank mates may not get their share of food in an aquarium with tiger barbs, slowly starving. Meanwhile, tiger barbs should never share an aquarium with long-finned fish like freshwater angelfish or bettas. Even in a large school -- which typically defuses their aggression -- fish with long-flowing fins are just too much of an attractive nuisance for tiger barbs to leave alone. Constant fin-nipping can lead to fin rot, a potentially lethal infection.