Yeah, pretty much without knowing what type of fish is the abuser, or what fish is being abused, or the size of your tank, or the other tankmates, or how much cover you have, it is hard to offer you advice.
Some fish species are naturally fin nippers and aggressive. Tiger barbs are the most notorious. Unless they are in schools of 7+ they usually gang up and harass tankmates, nipping fins and poking out eyes, usually to the death of their tankmates. A tank with tiger barbs must be quite big to support a school of 7+ as well as very highly decorated to give tankmates a place to flee. Even then, it doesn't garuntee that one or two of the barbs won't become bullies.
Cichlids are also pretty notorious as aggressors. Territorial cichlids placed in a tropical community tank will kill the less aggressive fish. Cichlids require large tanks to feel comfortable in. They are a type of fish that likes lots of personal space.
Blind cave tetras are also a nasty notorious nippers. I've seen them even harass plecos. It is a feeding mechanism. Since they are blind, anything they sense that appears to be food gets a nibble.
Even "docile" fish can become aggressive nippers under certain situations. Mollies, swordtail and guppies that are overcrowded will dominate and kill tankmates. Males or females in breeding condition may get a bit ornery. Fish that are not kept in the proper schooling numbers, or kept with too little decor hiding spaces will chase each other mercilessly. So as you can see, lots of variables are involved in fish agression, or even stress-related aggression. So please, fill us in as to what fish and what type of tank/community you have.
~~Colesea