When trying to deal effectively with aggression you must look at the basic causes of it. Some of these may be incompatible tank mates, spawning, or territorial issues. Spawning can bring on some of the deepest aggression in fish, most often to a third party fish in the tank. If this is the case, remove the non spawning fish from the aquarium if possible. When tank mates are chosen that are not compatible, there's a good chance they will fight till the death of one, be very cautious when mixing fish. Try to keep aggression levels the same, as well as species sizes. Is your tank too small? This can often bring on the battle as well, so ensure that you have adequate tank space to house your fish. Whichever the reason for the aggression levels, here are some things you might consider to try to reduce aggression levels.
Rearrange your aquarium - The theory behind this is that you rearrange plants, driftwood and other decorations in an effort to disorient the fish, and break established territories. I've found that this seldom works, as most often it's a territorial issue to begin with. You can expect some form of peace for up to two to three days before the aggression cycle begins again. This may work in some instances, anything is worth a try.
Tank divider - Sometimes it may be necessary to separate the fish using a divider. Be sure that the divider is inserted securely or the fish may break through this barrier. After a few days, when the fish seem calmer, remove the divider and see what happens. If they still insist on fighting, replace the divider. You may have to do this several times.
Reintroduce the fish - You need to have another aquarium for this one. Remove the aggressor from the tank to allow the weaker of the fish time to establish it's territory. You may want to place a definite territory boundary in the center of the tank such as dense plants, or a rocky structure. Provide hiding places on both sides of the boundary. Reintroduce the aggressor. If you do have a spare tank of adequate size, it's best to split them up as a last resort.
Dither fish - The idea behind this is to divert attention and aggression by purchasing several smaller, quick swimming fish into the aquarium. This is considered to be cruel by some, and you may lose some fish. If you choose to try this, provide lot's of hiding places for the dithers such as densely planted area's, caves, etc. This can be a rather effective way of decreasing aggression in some cases.
Returning a fish - When all else fails, you will have to return one of your fish, or purchase an aquarium for it. Some fish can be so aggressive that they will kill any and all tank mates and are best left to live a solitary life.