Aggression among fish can sometimes be a problem and some types of fish such as the cichlids have a reputation as aggressive and unsuitable for community aquariums. This is true for some species but there are still many species that can be appropriate for a community tank. The trick is to decorate your aquarium as to minimize aggression among the fish and the truth is that how you decorate your aquarium will have a huge impact on the level of your fish aggression.
So how do you decorate to minimize aggression? Simple, you do it by decorating the aquarium so that the fish can form territories with natural borders. An aquarium with little decoration and lots of open space makes it hard to form territories which usually leads to one fish trying to take the entire tank as his territory. If the fish, on the other hand, have natural borders they can form several territories and the aggression can usually be contained to the occasional border conflict which most often consists if the fish flaring at each other.
The first thing to consider is the line of sight. Reducing the line of sight so that the fish can’t always see each other regardless of where they are in the aquarium will reduce the level of aggression dramatically. Out of sight is out of mind when it comes to fish. This can be done by placing a few large plants in well-chosen spots. A large plant anywhere in the middle of the aquarium will, for example, reduce sight from one side of the aquarium to the other and hence reducing aggression.
Limiting line of site is a start but to minimize aggression you will as earlier mentioned also have to create natural territorial borders in the aquarium. How you do this depends on the kind of fish you keep and their behavior. A Gourami will as an example spend a large part of its time in the upper portion of the aquarium and a group of floating plants can then be a very good territorial border. The floating plants will not only serve as a border but will also help to further limit the line of site preventing the fish from seeing each other all the time.
Adding a group of floating plants will however not create a natural territorial border for fish that spend most of their time in the lower water layers such as different cichlids species. You will in this case instead have to create natural territorial borders on the bottom of the aquarium. This can be done by laying a piece of bogwood across the middle of the aquarium from the bottom to the front. The root will then serve as a natural territorial border and make it easy for your fish to establish that this site of the bogwood is mine, that side is yours. The same outcomes can be achieved by placing rocks or other decorations in a similar fashion. Similarly, you can divide the larger tank into more than the two territories.
It is also important that you make sure that both sides of the border contain everything the fish needs in their territory including suitable hiding and spawning places. If the fish is a cave dweller make sure that they have a cave or even better several in each territory.