Mismatching Your Species of Fish
Fish owners have lots to say about the personality of their pets. Before adding multiple species to your tank, connect with fellow fish hobbyists who can attest to whether two species will live together peacefully.
There are also charts available to help you mix and match species based on the level of aggression. If you happen to like an aggressive species of fish, such as Oscars or African cichlids, you will need to select tank mates that can tolerate aggression, usually others of the same species. One important exception is the betta fish, also known as Siamese fighting fish—and for good reason. This species should never be housed together. Two males put together will fight to the death. Even a male-female pairing should be under supervision and for the duration of breeding only, since they also will begin to fight.
Mixing aggressive fish with passive fish can lead to bullying. Fish that are too frightened to come out of hiding during feeding time will eventually starve to death. Avoid making impulsive purchases of fish and plan your tank selections in advance with research.