A rumble fish is a betta, or Siamese fighting fish. It is symbolic of the difficult situation that the boys in the story find themselves in, destined for conflict.
When Rusty-James sees the Siamese fighting fish in the store, it is a not so subtle connection to his own fate.
If you leaned a mirror against the bowl they'd kill themselves fighting their own reflection…
The betta fish will fight themselves while thinking they are fighting someone else. Symbolically, they are the same situation. They fight because they only know how to fight. The boys are the same way. They fight for silly, self-destructive reasons. Like the tiny fish in the bowl, the characters in the story are trapped.