I tried to introduce a female guppy into the tank with my male betta Alfy. I kept her for 2 weeks in a quarantine tank and all seemed fine so I moved her into the tank with Alfy. He chased her all over the tank, flaring at her, trying to attack her. After less than 10 minutes I had to net the guppy and move her back into the quarantine tank so Alfy didn't kill her. I know someone at work who has a tank of guppies, so I was going to ask her if she wanted another, but the guppy died before I could talk to my coworker.
The betta was, and still is, perfectly fine. I learned a lesson, Alfy is perfectly happy being alone in his tank. Trying to add another fish was admittedly for me, not him. I mistakenly thought Alfy and the guppy wouldn't care about each other, and that the tank would look nicer with more than one fish. Something I'm not going to try again.