Who Gives A Fish Forums

I'm no expert, but that quarantine tank seems a bit cramped for a betta. Ideally, you'd want something with at least 2-5 gallons, a heater, and a filter to keep the water quality stable. The fin rot and those white dots around her fins might be signs of poor water conditions and possibly ick ...

Your tank is a step up from a bowl, I'll give you that, but it does need to be cycled. I'm not convinced it's fin rot, though - just keep an eye on it and treat if it gets worse. When doing water changes, make sure to dechlorinate the new water.