Thank you for your reply! I am gonna upgrade in a few months most likely. Until then I am gonna go 20% water changes every 3 days. Hopefully, he's gonna do well and recover. He is really active and responsiveIt's hard to tell from the images if it is rot.
From experience, the fins tend to curl when water quality is low. In a bowl this is much more likely to happen quickly. Bettas really do much better in something bigger with filtration. Some of this damage definitely happened due to the situation he was in before you got him, but upgrading him will really help him recover quickly and live a much longer, healthier and happy life.
Ok. Thank you very much for your response! I was really getting worried. I can't wait to upgrade his tank and see him swim happily in his larger homeIt doesn't look like rot. Rot is a fungus and will look cottony or fuzzy. It just looks like damage from living in a cup.