Remember that bettas are different and have different aggression levels. Some bettas are more aggressive than others. So base your tankmates accordingly. I wouldnt put any other fish in the tank unless it is 10 gallons or less. And a fitler is present. Some people say to avoid fishes with long fins and bright colors, but i havent found any truth to this. Just remember that all bettas are different. Common betta tankmates include, dwarf otto catfish, white clouds, and other small comunity fish. (See Betta Buddies for more info)