A mistake made by many inexperienced aquarium owners is to accidentally purchase aggressive tropical fish and add them to their beautiful family of fish. Suddenly the aquarium goes from being quiet and calming, to a snack bar and buffet.
Aggressive tropical fish do not always attempt to eat the other fish, but will always be the aggressor in the tank. They may bite off parts of fins, bump, or simply chase the other fish incessantly.
While seeing all the added activity may be fun at first, you will quickly start seeing your calm fish becoming agitated, injured, and unhealthy. You do not want to mix aggressive and non-aggressive fish in the same tank.
The most common aggressive fish include the Red Oscar, Jack Dempsey, Butterfly Fish, Black Paradisefish, and of course the red belly Piranha. These fish are often purchased because of their beauty, without taking the time to learn about their personality.
As an aquarium owner, you should always keep an inventory of the types of fish you have in your tank. When you go out to buy more fish take your inventory with you, and discuss options with the people in the pet shop about which fish will work in your mix. Most shops are very responsible and will even have small signs on the tanks of aggressive fish to warn you.
As an example, if you want to own the beautiful Red Oscar fish you need to really plan ahead. The fish will require a tank of their own. You can only put in other Red Oscars, and they all need to be approximately the same size. If you put in smaller fish they will be the target of the larger fish.
Even the seemingly innocent Betta Fish needs to be treated with caution. You need to be careful to never put two males in one tank or you will learn the meaning of aggressive tropical fish in a moment. The two males will go to battle and not stop until one is floating on the surface.
In some situations, you can mix a few semi-aggressive fish with aggressive fish if they are all the same size. Use caution when attempting this and have another tank prepared for moving fish if things go badly. You may not know immediately, but keep a close eye on tails and fins of the fish to make sure they are not going to battle.
Owning aggressive tropical fish can be great fun. Owning Piranha and watching them voraciously go after their food is enjoyed by many people. When you buy fish with the knowledge of what you are getting and how to handle them you will have great luck. It only becomes a problem when you ignore the signs and combine your aggressive fish with other fish they see as their lunch. Use common sense and keep your aggressive fish in a special tank, and enjoy watching them mix with their own species.