I know we get a lot of questions about putting bettas with other fish, and lately we've had a few more members getting imbellises, so I thought it might be helpful for me to post this little warning, as it involves both.
As you may know, the word imbellis means "peaceful." Betta imbellis is said to be less aggressive then other species (ie., [/i]Betta splendens[/i]) and multiple males have been reported to live in harmony together. These were the thoughts that resonated in my head as I decided to put my imbellis, Titan, in the 5 gallon tank that used to house my splendens, Lancelot, who recently passed away. The tank is also home to 3 very small cory cats. These little charmers are the most peaceful, least aggressive fish you could ask for. They just bounce around, doing their own thing, and wouldn't hurt a fly. They and my splendens lived together for many months, quite happily ignoring each other.
As soon as I placed my imbellis into the tank, he decided the cory cave should be his, and started chasing the cats. But he never nipped at them, and he quickly got bored and stopped. He was definitely nervous and jumpy in his new surroundings, so I figured he'd like to be left alone for a while.
I came back a couple hours later and found one of my tiny cories' tails was half missing. A couple minutes later, I saw my "peaceful betta" swimming towards me with the cory in his mouth. When he saw me, he dropped the cory, who swam away grateful to avoid becoming the main course. Luckily I'd left the betta's old tank set up, so I quickly netted him and tossed him back in.
The morals of this story are these.
A) My splendens, who is of a more aggressive species did not do well alone (he bit his tail) but loved the company of the cories. My imbellis, who is of a more peaceful species almost ate one of my cories. Every betta is different and just because one betta does well with tankmates, doesn't mean another will.
B) Always have a back-up plan. If I hadn't had somewhere to stick my betta, I suspect the cory would have become dinner.