Fish incompatible with betta fish

From Betta Fish Wiki

• Very small fish (smaller than one inch) may be eaten

• Fish with long, flowing fins may trigger aggression

• Slow-swimming fish, e.g. fancy guppies, will be unable to escape bullying

• Mollies tend to bite the fins off and have been known to kill bettas

• Schooling fish, especially barbs and certain tetras, will nip at bettas' fins

• Aggressive fish, such as piranhas, and bluegills, will bully or (if large enough) eat bettas

• Fish belonging to the same biological family as the betta, such as Paradise Fish and gouramis, may attack or be attacked due to their relatively similar appearance and cross species aggression

• Goldfish are unsuitable tank-mates because of their great appetites, preference for cold water, and high excretion-rate. Like many tropical fish, Betta splendens might harass and kill small, slow fancy goldfish; yet, goldfish have been known to bite the Betta's tail.