The "Rumble Fish" that this question refers to is asking about the Siamese fighting fish that the Motorcycle Boy likes so much from Mr. Dobson's pet store. The Siamese fighting fish have to be kept apart from each other because otherwise they would fight to the death:
They try to kill each other. If you leaned a mirror against the bowl they'd kill themselves trying to fight their own reflection.
The fish and their behavior are symbolic of the warring gang members and their constant affinity toward using violence against one another. Rusty-James and the gang members are like the fighting fish. He is a gang member, and he fights different gang members simply because they exist and are within sight of him and his territory. It's what he does, and it's all that he knows how to do. It's a part of genetic fiber in the same way that the fish have no other outlook on life. It's normal.
The Rumble Fish are tied to another symbol in the book too. The Motorcycle Boy is curious if the fish would act that way if they were given a new environment with additional space from other fighting fish:
"Wonder if they'd act that way in the river," the Motorcycle Boy went on.
The Motorcycle Boy eventually steals the fish and releases them into the river. He hopes that the new environment and freedom will result in a behavioral change. The fish store is captivity, and the river is freedom. This is essentially what happened to the Motorcycle Boy when he spent time away from the town and the violence.