When you create a picture of an aquarium in your mind, most likely it will be tranquil and serene. After all, most aquarium enthusiasts maintain an aquarium precisely to have a beautiful and inspirational bit of nature indoors– a source of passive beauty. This of course is accomplished by having a mix of fish that are compatible with each other. If you have a freshwater aquarium you must be versed in the temperament of aggressive freshwater fish species in order to keep the peace.
Before we look at the most common species known for their harsh temperament, it must be prefaced with the following. All fish will be aggressive and all fish will be passive. This may sound like a contradictory statement, but it is essentially a law of nature. Until one starts keeping great white sharks in a home aquarium, every species that you will have in your aquarium will be both a predator and prey of other species in nature. Yes, even your plant eaters prey on plants! Being aware of this natural occurrence will help you in avoiding Wild West style saloon fights, "fish-style."
Now, who are the toughest of the tough … the meanest of the mean in aquarium freshwater fish?
Red Bellied Piranha
When it comes to aggressive freshwater fish, you can not get more aggressive than the red belly piranha. They can make for great aquarium fish when kept with others of their own kind. They are large, growing as big as 12 inches and will eat fish smaller than them, so contemplating an introduction of red belly piranha into a cross-species tank would be a mistake.
Siamese Fighting Fish
This is the species most commonly labeled as an aggressive freshwater fish. This reputation is well deserved, but it should be tempered slightly. First, male Siamese fighting fish, or Betta fish as they are also known, are very aggressive. If two males are placed in the same tank they will fight to the death. This is how they establish mating and feeding territories in nature. Obviously, you would want to avoid this situation. Female Betta fish do not display this behavior and can live communally.
Male Bettas could be placed in a tank inhabited by other species, but even here caution should be taken. If your other fish display long wavy appendages similar to a Betta or if the tank is crowded, fights could break out which can be deadly for other less aggressive species.
Do not let their common name fool you. These cichlids can be as aggressive as they are beautiful. They can coexist in a tank with tetras fairly well, as the tetra is rather docile with the angelfish and the angelfish's mouth does accommodate a tetra to well for biting or killing purposes. Guppies, however, do not generally mix well with angelfish. The guppy has a tendency to nip at the angelfish's extended fins and due to their small size, often resulting in seriously injured or dead guppies by the mouth of an angelfish.
A good rule of thumb with angelfish is to avoid placing them with smaller species. When new fish are introduced into an aquarium environment with angelfish present, it is best to do so after feeding time and preferably under low-light or dark conditions.
Males of this species can be aggressive, but tend to temper their aggression once a hierarchy has been established within the aquarium population. Females will not display aggressiveness.
As was stated at the beginning all fish can be aggressive and all can be passive. It is just a matter of you, the aquarium keeper, being aware of each specific fish species' temperament before blindly adding a new member to your tank.
Always keep in mind that your fish selection should not be based entirely on how beautiful a fish looks. Never abandon the temperament and aggressiveness comparability factor. As long as you do this, even the so called aggressive freshwater fish can be beautiful additions to your aquarium.