Fish can be a bit territorial and try to show who is the boss.
My old fish used to do this to each other so i went out and bought the biggest plastic fish i could find and put that in and the *boss* fish didnt like it but he soon settled hehe.
Now i just have that problem with my pet rats......both female and they playfight but they squeak and it terrifys me as it sounds worse than it is! little sods!
