Join Date: Aug 2012
Location: Minneapolis, Minnesota
Totally depends on the fish. I have enough bettas right now to show you one that wouldn't harm a hair on a fly's head; and one that will savagely attack any living thing that has the misfortune of landing in the tank. And that's of each sex, too.
I also have one girl that I removed from my sorority and she actually became depressed because she didn't like being alone. Her previously vibrant purple ventrals and caudal tips turned white when she was alone. After a week of her moping, I slipped her back into the sorority tank. She perked right up. Another girl became depressed when I put her in the sorority tank and had to be removed because she was mopey about having to share a tank with other fish XD
It's always important to advise people that any tankmate could be killed by a betta. Even one that seems docile for a long time may one day turn on tankmates. But I do believe that there is a wide range of personalities so each fish should be considered on that basis.